Wrap get and set interface

[dougbrn]

Feature request
We should update Nested-Dask to use the get and set interface introduced in lincc-frameworks/nested-pandas#135

This will involve overwriting the __getitem__ and __setitem__ functions. I'm not sure that using map_partitions over the nested_pandas __getitem__ and __setitem__ would be correct, or to just reimplement the functionality within the nested-dask nestedframe get/set functions.

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