add enmap reader

[antoineba]

[antoineba]

The EnMAP RPC have 1 model per band. Thus the parsing function of the xml returns a dict containing all models, with the keys corresponding to the bands numbers. The parsing function works for L1B,L1C and L2A products (only L1B product example is given in the test). The associated test check that here are multiple RPC models and generate an RPCModel object for the first band.