Include tables module in required libraries
Using `to_hdf` method without the tables module installed may results in infinite recursion crash: Fatal Python error: Cannot recover from stack overflow.

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

huh? this is an optional dependency.  All right, I understand.