Add explicit grid destroy when deleting Regridder

[aulemahal]

Fixes JiaweiZhuang/xESMF#53.

When the Regridder or SpatialAverager object is garage-collected, the ESMpy grid objects are not destroyed as they seem to still be referenced somewhere in the ESMpy Manager (or something alike). This change explicitly destroys the grids when the Regridder object is removed.

This will break any code that makes use of the Grid objects in a scope where the Regridder itself is not referenced. For example:

import xesmf as xe

def get_grid(ds1, ds2):
    reg = xe.Regridder(ds, ds2)
    return reg.grid_in    

Will most likely return a destroyed grid object.

The other solution I see is to make a destroy method on the Regridder object that destroys underlying grids, but that demands an explicit action from the user. It won't really fix the "memory leak" feeling that users get.