API:  for HDFStore, add the keyword dropna=True to append to change whether ALL nan rows are not written
      to the store (default is True, ALL nan rows are NOT written), also settable
      via the option io.hdf.dropna_table (GH4625)
Merge pull request #4714 from jreback/hdf_nan

API:  for HDFStore, add the keyword dropna=True to append to change whether to write ALL nan rows to the store (GH4625)
BUG: HDFStore.append_to_multiple - ensures rows are synchronized before writing

adds dropna kwarg + docstring + tests + documentation + release note

python3 compatible