Harmonized layers for biopar/indices

[jdries]

Both Terrascope and sentinelhub have different ways to compute various indices like fapar/fcover/ndvi/...
As a user, I do not always want to write complex graphs to construct these common indices, but rather want the backend to select the easiest option.

We can consider to add a SENTINEL2_LEVEL3 layer, with bands for various popular indices.
The backend can then choose to either compute them on Sentinelhub, or load a precomputed layer available at Terrascope.