CLN: Rename "add" to "sum" in groupby

[rhshadrach]

• closes #xxxx (Replace xxxx with the Github issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

sum is the one method whose name doesn't agree with the corresponding how

[attack68]

When looking at this it reminded me of Pokemon: 'are you sure you caught them all?'

[rhshadrach]

@attack68 - I think our test coverage is sufficient to feel good I got all the code changes. For comments, I would guess it's possible I missed some. I used grep -nr --include \*.p* "add" in both the groupby and the _libs directories and went through the results (and just double checked now).

[mroeschke]

Thanks @rhshadrach