Merge branch 'develop' into issue75-mappers
#75: remove namespace package nansat.mappers;  move import_mappers to nansat.tools; make nansatMappers global in nansat.py; make all import absolute; in mapper_ncep_wind_online move import Nansat into __init__;
#75: lazy import of nansat.mappers and nansat_mappers
#75: move import Nansat to top in mapper_ncep_wind_online
#75: fix minor bug and add logging
#75 move _import_mappers out of Nansat back to nansat.nansat to be usable in test_nansat; add try/except ImportError in _import_mappers
#75: make _import_mappers() strict; make mapper_opendap flexible
#75 adjust warning messages
#75 add MapperImportError
#75 put MapperImportError in nansatMappers instead of a non-importable mapper
#75 remove MapperImportError; simplify mapper_opendap;
#75 show correct stack trace when raising error about non importable mapper
#75 SHow ImportError warnings before using mapper_generic
#75 make WrongMapperError empty
Merge branch 'issue75-mappers' into develop