MVP: Implement the calc_nobs utility function #13
Description
Feature request
Blocks #9. To demonstrate how more-domain specific functions can be built and interact with the core package, we should implement calc_nobs in a separate utility module (or utils of NestedFrame). The implementation should follow TAPE: https://tape.readthedocs.io/en/latest/autoapi/tape/index.html#tape.Ensemble.calc_nobs, and usage should look like this:
from nested_frame.utils import calc_nobs
nf = calc_nobs(nf, "my_nested", by="band")
Because we don't have column intuition via column mapping (a la TAPE), I think it's best to have a "by" kwarg that groups by unique values of the given column and returns columns for each group. Potentially annoying if a user chooses the wrong column as this could produce a lot of columns.
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