ENH: correlation function accepts method being a callable
- other than the listed strings for the `method` argument, accept a callable for generic correlation calculations
- ~~~minor fix of = to == in requirements file~~~

- [ ] closes #xxxx (no issue opened for PR)
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry (not sure if I should do this or the person merging this PR)
