Add a new preprocessor to extend the functionality

[geek-yang]

Following the suggestions from PR #152, we need a new preprocessor to provide the following options to the user:

• spatial selection (so selecting a certain spatial extent with lat and lon)
• regridding
• apply land/sea mask

Hint for implementation:

• spatial selection da.sel(lat=slice(20, 30), lon=slice(110, 140))
• for regridding, we can do simple interpolation with da.interp() or da.interp_like(), and for complex ones with xesmf (e.g. regridder = xe.Regridder(ds, ds_out, "bilinear") and then ds_out = regridder(ds))
• apply land/sea mask da.where(np.isnan(mask))

And more importantly, as a user, I would need a preprocessor that can deal with NaN in different ways (e.g. interpolation, mean, etc).