HDFStore.walk() to iterate on groups
This PR adds a walk() method to HDFStore in order to iterate on groups.

This is a revival of the initial PR #10932, rebased on upstream/master and updated tests.

- [x] closes #10143 
- [x] tests added / passed: test_pytables.py:testwalk()
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
