#67 import NSR in vrt.py
#67 add NSR() to nansat.py where necessary
#67 raise errors in NSR() if no valid projection is provided
#67 add NSR to vrt.py. Fix non-package-style imports
#67 fix non-package-style imports in all mappers
#67 add NSR to test_nansatshape and fix bug in Nansat.get_transec()
#67 fix few bugs in Nansat.get_transect()