Change isochrone renderer to QgsGraduatedSymbolRenderer to support more than 10 colors

[merydian]

To log the correct ApiError message in the isochrone processing algorithm window, when there are more than 10 ranges inputted.

Otherwise this will be printed:

File "/home/adroit/.local/share/QGIS/QGIS3/profiles/default/python/plugins/ORStools/proc/isochrones_layer_proc.py", line 218, in postProcessAlgorithm
self.isochrones.stylePoly(processed_layer)
File "/home/adroit/.local/share/QGIS/QGIS3/profiles/default/python/plugins/ORStools/common/isochrones_core.py", line 208, in stylePoly
color=colors[cid], strokeColor=QColor("#000000")
KeyError: 10```

[koebi]

I think the limit of 10 ranges only exists for the public API.
For this, we should see that a useful error is reported when trying more than 10 isochrones.
For everything else, we should implement some sort of cycling through colors, or using random colors when over 10.

[merydian]

Current implementation uses QgsGraduatedSymbolRenderer with Spectral color ramp. The amount of possible ranges is no longer limited like this.

20 ranges: