Handle partial_bottom_cells=.true. in xgcm_util.py module

[andersy005]

When partial_bottom_cells=.true. in POP, the bottom layer doesn't always have the nominal thickness. We probably want this utility to handle that use case, assuming that xgcm can handle it (which I think is a reasonable assumption). I think this can be detected in model output by comparing HT to something like z_w[KMT] (I might have an off-by-1 error here). If they differ in some places, then partial_bottom_cells=.true..

Originally posted by @klindsay28 in #21 (comment)