Allow users to select color palette on ncWMS2 endpoints

[rsignell-usgs]

It would be useful to allow users to select the color palette on ncWMS2 endpoints, as shown here on the Godiva3 ncWMS2 client:

On ncWMS2 endpoints, the palettes are not included as styles, but are contained in the GetCapabilities abstract. This would be nasty to parse, but ncWMS2 already supplies an easy way to retrieve this information, via a GetMetadata&item=layerDetails` extension. For example, this request:
http://geoport.whoi.edu/ncWMS2/wms?request=GetMetadata&item=layerDetails&layerName=FVCOM-NECOFS-GOM3/temp
returns this simple JSON structure with the palette info:

{
    "units": "degrees_C",
    "bbox":     [
        -75.68433380126953,
        35.27653121948242,
        -56.850791931152344,
        46.14595031738281
    ],
    "scaleRange":     [
        -1.931,
        21.93
    ],
    "numColorBands": 250,
    "supportedStyles":     [
        "default-scalar",
        "colored_contours",
        "contours",
        "raster"
    ],
    "noPaletteStyles": ["contours"],
    "categorical": false,
    "queryable": true,
    "downloadable": true,
    "continuousZ": false,
    "zaxis":     {
        "units": "level",
        "positive": true,
        "values":         [
            0,
            1,
            2,
            3,
            4,
            5,
            6,
            7,
            8,
            9,
            10,
            11,
            12,
            13,
            14,
            15,
            16,
            17,
            18,
            19,
            20,
            21,
            22,
            23,
            24,
            25,
            26,
            27,
            28,
            29,
            30,
            31,
            32,
            33,
            34,
            35,
            36,
            37,
            38,
            39
        ]
    },
    "continuousT": false,
    "datesWithData": {"2016": {"10":     [
        1,
        2,
        3,
        4,
        5,
        6,
        7
    ]}},
    "timeAxisUnits": "ISO8601",
    "supportsTimeseries": true,
    "supportsProfiles": true,
    "supportsTransects": true,
    "nearestTimeIso": "2016-11-04T10:58:07.500Z",
    "moreInfo": "",
    "copyright": "",
    "palettes":     [
        "default",
        "default-inv",
        "div-BrBG",
        "div-BrBG-inv",
        "div-BuRd",
        "div-BuRd-inv",
        "div-BuRd2",
        "div-BuRd2-inv",
        "div-PRGn",
        "div-PRGn-inv",
        "div-PiYG",
        "div-PiYG-inv",
        "div-PuOr",
        "div-PuOr-inv",
        "div-RdBu",
        "div-RdBu-inv",
        "div-RdGy",
        "div-RdGy-inv",
        "div-RdYlBu",
        "div-RdYlBu-inv",
        "div-RdYlGn",
        "div-RdYlGn-inv",
        "div-Spectral",
        "div-Spectral-inv",
        "psu-inferno",
        "psu-inferno-inv",
        "psu-magma",
        "psu-magma-inv",
        "psu-plasma",
        "psu-plasma-inv",
        "psu-viridis",
        "psu-viridis-inv",
        "seq-BkBu",
        "seq-BkBu-inv",
        "seq-BkGn",
        "seq-BkGn-inv",
        "seq-BkRd",
        "seq-BkRd-inv",
        "seq-BkYl",
        "seq-BkYl-inv",
        "seq-BlueHeat",
        "seq-BlueHeat-inv",
        "seq-Blues",
        "seq-Blues-inv",
        "seq-BuGn",
        "seq-BuGn-inv",
        "seq-BuPu",
        "seq-BuPu-inv",
        "seq-BuYl",
        "seq-BuYl-inv",
        "seq-GnBu",
        "seq-GnBu-inv",
        "seq-Greens",
        "seq-Greens-inv",
        "seq-Greys",
        "seq-Greys-inv",
        "seq-GreysRev",
        "seq-GreysRev-inv",
        "seq-Heat",
        "seq-Heat-inv",
        "seq-OrRd",
        "seq-OrRd-inv",
        "seq-Oranges",
        "seq-Oranges-inv",
        "seq-PuBu",
        "seq-PuBu-inv",
        "seq-PuBuGn",
        "seq-PuBuGn-inv",
        "seq-PuRd",
        "seq-PuRd-inv",
        "seq-Purples",
        "seq-Purples-inv",
        "seq-RdPu",
        "seq-RdPu-inv",
        "seq-Reds",
        "seq-Reds-inv",
        "seq-YlGn",
        "seq-YlGn-inv",
        "seq-YlGnBu",
        "seq-YlGnBu-inv",
        "seq-YlOrBr",
        "seq-YlOrBr-inv",
        "seq-YlOrRd",
        "seq-YlOrRd-inv",
        "seq-cubeYF",
        "seq-cubeYF-inv",
        "x-Ncview",
        "x-Ncview-inv",
        "x-Occam",
        "x-Occam-inv",
        "x-Rainbow",
        "x-Rainbow-inv",
        "x-Sst",
        "x-Sst-inv"
    ],
    "defaultPalette": "default",
    "aboveMaxColor": "#FF000000",
    "belowMinColor": "#FF000000",
    "noDataColor": "transparent",
    "logScaling": false
}

The palette selected, for example x-Sst is then used in subsequent requests, like:
http://geoport.whoi.edu/ncWMS2/wms?FORMAT=image%2Fpng&TRANSPARENT=TRUE&STYLES=default-scalar%2Fx-Sst&LAYERS=FVCOM-NECOFS-GOM3%2Ftemp&TIME=2016-11-04T10%3A58%3A07.500Z&ELEVATION=0&COLORSCALERANGE=10.73%2C17.1&NUMCOLORBANDS=250&ABOVEMAXCOLOR=0x000000&BELOWMINCOLOR=0x000000&BGCOLOR=transparent&LOGSCALE=false&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&SRS=EPSG%3A4326&BBOX=-72.420933854153,39.933157812517,-71.023803124986,41.330288541684&WIDTH=256&HEIGHT=256

[kring]

If ncWMS2 reported palettes as a dimension, this would work automatically. But with the current setup, it will need to be a ncWMS2-specific feature.

[rsignell-usgs]

@guygriffiths, any chance of getting this working?
I guess responding on Reading-eScience-Centre/edal-java#86 would be better?

[guygriffiths]

@rsignell-usgs - The palettes are advertised as styles, it's just that only a subset of them are listed. Implemented palettes as another dimension could be done, but it's not a trivial task.