move panel-specific swaplevel to panel
NDFrame.swaplevel is overriden by both Series and DataFrame, so in practice the "generic" implementation is specific to Panel.  This PR just moves the method from NDFrame to Panel.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
