TST: test for GH4633, bool(obj) behavior

BUG: GH4633, rever to GH1073, whereby __nonzero__ always raises for all NDFrame objects
DOC: docs to add detail on gotchas for true value testing
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)
API/DOC: in HDFStore, change fmt -> format, change Storer format to Fixed format (just a name change);
         option for format is now fixed(f) or table(t)
API: add io.hdf.default_format option to enable an override to default storage formats in HDFStore
Merge pull request #4715 from jreback/hdf_format

API: change nomeclature in HDFStore to use format=fixed(f) | table(t)
Merge pull request #4657 from jreback/bool

API: GH4633, bool(obj) behavior, raise on __nonzero__ always
API/BUG: a column multi-index will be recreated properly (GH4710)

API: raise on trying to use a multi-index with data_columns on the same axis
Merge pull request #4716 from jreback/hdf_column_mi

API/BUG: a column multi-index will be recreated properly (GH4710)