issue84-use-tps Add option 'skip_gcps' for faster reprojection with TPS (at cost of accuracy) when source or destination image has GCPs. Default value may be set by mapper. Default value of 3 is set for ASAR mapper, reducing reprojection time by a factor of ~3, and invisible effect on accuracy (checked with landmask overlay)
#84 add tps and use_gcps options to Nansat.watermask() for convenience
#84 update Nansat.watermask() docstring
#84 merge issue84-use-tps into develop