Adopt to alternative WMTS sources

[civilizemars]

Context

The simple-globe APIs gave me 403 errors and I can't get an API key.

So I wanted to test alternative WMTS sources, specifially Mars maps from NASA:

My question is, how I could adopt itowns to work with the {z}/{y}/{x} Maptile format.

https://api.nasa.gov/mars-wmts/catalog/Mars_Viking_MDIM21_ClrMosaic_global_232m/1.0.0//default/default028mm/{z}/{y}/{x}.jpg?api_key=abcdefghi

[gchoqueux]

The api_key url param isn't necessary. This source isn't WTMSSource, use TMSSource instead of.

var marsSource = new itowns.TMSSource({
    // crs: 'EPSG:104905',
    crs: 'EPSG:4326',
    url : 'https://api.nasa.gov/mars-wmts/catalog/Mars_Viking_MDIM21_ClrMosaic_global_232m/1.0.0//default/default028mm/${z}/${y}/${x}.jpg' });

var marsLayer = new itowns.ColorLayer('wms_imagery', {
    source: marsSource,
});

view.addLayer(marsLayer);

With other Atmosphere parameter:

[gchoqueux]

I add a this mars example in this PR #1610

[gchoqueux]

Mars example added!
http://www.itowns-project.org/itowns/examples/#mars