DEPR: disallow mutating operations in groupby.apply

[jbrockmendel]

A common issue [citation needed] comes up in groupby.apply when a user passes a function that modifies its argument in-place. This is because we call the function on the first group first to see if we can use a fastpath, and then again when we apply it to all groups.

We could add a kwarg to specify if a function may mutate its data, e.g. gb.apply(myfunc, may_mutate=True) in which case we could avoid re-calling the function (probably means no fast-path). With may_mutate=False we could use a context in which we set the data's flags to be immutable before calling anything on it.

[jreback]

dask has the keyword pure for this purpose
we could adopt it

cc @TomAugspurger
cc @mrocklin

[mrocklin]

In hindsight the term pure was a bad choice for us. It means lots of things, not all of which you may intend to communicate here.

[mroeschke]

Looks like this is a duplicate of #12653