adjust/partialize by selected column

[mattansb]

Currently I can only partialize by all the "other" selected columns (the ones that aren't part of the correlation).

It would be nice to be able to "fix" which columns I want to partialize by.

For example, I can get multi-level correlations this way:

correlation::correlation(
  iris,
  partial_by = "Species", # <<<<<<<<<<<<
  partial = TRUE,
  multilevel = TRUE
)

[DominiqueMakowski]

rather than adding a new arg partial_by I'd just allow partial to take in characters vectors:

correlation::correlation(iris, partial = "Species", multilevel = TRUE)

[jmgirard]

This was closed but didn't seem to have been implemented.

[mattansb]

Closing in favor of #311