ENH: Added multicolumn/multirow support for latex
- [x] closes #13508
- [X] tests added / passed
- [X] passes `git diff upstream/master | flake8 --diff`
- [X] whatsnew entry

Print names of MultiIndex columns.
Added "multicol" and "multirow" flags to to_latex
which trigger the corresponding feature.
Multirow adds clines to visually separate sections.
