Add documentation for ProcessingDataFusionLayer

[zcernigoj]

e.g.

ProcessingDataFusionLayer timespan

Sentinel Hub supports setting timespan for each datasource used in data fusion (docs, examples)

Sentinelhub.js does the same.

• timespan (fromTime, toTime) for each layer can be set when creating ProcessingDataFusionLayer layer
  • if the layer's timespan (fromTime, toTime) is set, it will be used in .getMap(), even if fromTime and toTime parameters are provided for .getMap()
  • if the layer's timespan (fromTime, toTime) is NOT set, .getMap() will use fromTime and toTime parameters provided to it
• if no timespans (fromTime, toTime) are set for the layers, fromTime and toTime params for .getMap() must be provided

relevant ProcessingDataFusionLayer code