Adding "allow_rechunk" option for variables derived from grid differences (#24)

* adding "allow_rechunk" option

this commit incorporates an optional "allow_rechunk" option for collect_budgets and budget_fill_dict

* adding test "test_budget_fill_dict_allow_rechunk" that tests the "allow_rechunk" option

* Clean linting and manually add chunks to small example dataset
By explicitly adding chunks to the example MOM6 dataset, we should
be able to better catch chunk-related problems in the future.

* Add `allow_chunk` to docstrings

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Co-authored-by: Henri Drake <hfdrake@uci.edu>

Author: anthony-meza

.gitignore

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 .ipynb_checkpoints
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