ASAR mapper now generates incidence angle and SAR look direction through small internal Nansat object, reprojected onto full ASAR domain. Takes care of wrapping problem around 0-360 degrees for SAR look direction.
Improved performance of calculation of incidence angle and SAR look direction, by avoiding Nansat.add_band() which created chain of VRTs, each requiring evaluation. Also using now vrt.get_resized_vrt() which seems to be more accurate than Nansat.reproject()
Radarsat2-mapper now adds band with SAR look direction, using same method as ASAR mapper.
Merge branch 'issue57-sar-azimuth' into develop

Conflicts:
	mappers/mapper_asar.py
Merged issue57-sar-azimuth into develop, manual fix of conflicts