add Pdata.Pdata method 'from_dataframe_groupby'
refind method for setting break axis
Big restructure of bmap code to make the data transform
and colorbar functions work as both functions for contourf map
and imshow map.
make gnc.Ncdata.map to use bmap.maptourf and bmap.mapimshow directly.
return the single value for ListSingleValue in pb.py