Added a way to check if a numeric type cell is NaN

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

is there an issue describing why the current behavior is wrong?  can you add a test? I am -1 on converting NA values to 0 - is there an open issue about this? @antonini thanks for the interest in submitting a PR! I don't know if this is a change we actually want to make though. If you feel strongly about it I'd suggest opening an issue first and can be discussed there