Enhancement/speed up geolayer generation

Merged Daniel Scheffler requested to merge enhancement/speed_up_geolayer_generation into master

Revised RPC_3D_Geolayer_Generator class to reduce computation time. Geolayers for the VNIR and SWIR are now computed within 2 seconds for the Arcachon test dataset (150 lines), i.e., ~130x faster than before.

Closes issue #41 (closed).

@segl @nbohn

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