added local shift-correction based on geometric quality grid; added README

COREG:
- calculate_spatial_shifts(): bugfix for printing None instead of calculated shifts within RuntimeError in case of too large shifts

DESHIFTER:
- updated documentation of keyword warp_alg
- implemented multiprocessing (keyword CPUs)
- implemented first prototype of local shift-correction based on a list of GCPs

Geom_Quality_Grid:
- replaced keyword 'multiproc' by 'CPUs'
- added method to_GCPList()
- added method correct_shifts()

README:
- added README for the whole package