CLN: use how="all" so that things that are not nan in other columns are kept
Merge pull request #4111 from cpcloud/data-dot-py-cleanup

CLN: use how="all" in data.py so that things that are not nan in other columns are kept
ENH: allow layout for grouped DataFrame histograms

Also remove the slow decorator for tests that take < 100 ms time
DOC: add release notes
TST: ensure things work with series
DOC: more release notes