BUG: fix pivot_table with multiple aggregations per column and margins=True #12210
Fix handling of list of functions per column:
    - in pivot_table (checking) column name conflict
    - in _compute_grand_margin

This case is still failing: if also specify columns='C', then there's another exception in
_generate_marginal_results.

 - [X] closes #12210
 - [X] tests added / passed
 - [ ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry
