Allow dict for methods in xarray.Dataset.interp

[andypbarrett]

I've been using interp to reproject and resample reanalysis data in lat-lon grids to projected coordinates. It works great!

However, I have one DataArray with categorical data (precipitation type) that is still numeric. This requires nearest-neighbour interpolation rather than (bi)linear interpolation. It would be nice to be able to do something like.

dataset.interp(latitude=new_lats, longitude=new_lons, method={'t2m': 'linear', 'ptype': 'nearest'})

Cheers

Andy