API: add ddof to expanding/rolling_cov()
Merge pull request #8280 from seth-p/rolling_cov_ddof

API: add ddof to expanding/rolling_cov()
BUG: boxplot fails when one column is all NaNs

Add test case for empty column

Replace empty arrays with [np.nan]

Add fix to the release notes

Add issue numbers
API: accept -1 for layout