Revised RPC_3D_Geolayer_Generator such that the geolayer computation is now only executed for bands with differing RPC coefficients. Since we only have one set of RPC coefficients per detector by now, this speeds up the geolayer generation by the factor 20-50. Added Test_RPC_3D_Geolayer_Generator class. Added natsort to dependencies.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>