Integrate shaded computation with saga-gis

Integrate saga-gis and shaded mask computation, using task 117 in #21 (closed)

• integrate saga_gis into environment on hpc and docker file
• include saga-gis code to compute shaded masm
• add configuration item to add path to saga-gis library and threshold
• add error management : if no path defined => compute only hillshade mask, if invalid path => error message