Re-added model_wind.py, and deleted download_NCEP_GFS.py
Added basic land mask to plot function.
Moved reprojection of wind from model_wind to sar_wind, otherwise ModelWind objects are recreated from filename when input til SARWind class. Set wind as first argument to ModelWind constructor, as filename or Nansat object are sufficient arguments, whereas time is not (must be accompanied by a domain)