BUG: NaN values not converted to Stata missing values

Stata does not correctly handle NaNs, and so these must be replaced with Stata
missing values (. by default).  The fix checks floating point columns for nan
and replaces these with the Stata numeric code for (.).  One of the code paths
which writes files correctly handled this case, and this last-minute check was
removed.

The write_index option was also being ignored by omission. This has been fixed
and numerous tests which were not correct have been fixed.

Also contains some additional tests which were uncovered edges cases related to
fix.
Merge pull request #6685 from bashtage/stata-world-indicators

BUG: NaN values not converted to Stata missing values (GH6684)
BUG: Bug in resample with extra bins when using an evenly divisible frequency (GH4076)
Merge pull request #6690 from jreback/resample_bin

BUG: Bug in resample with extra bins when using an evenly divisible freq (GH4076)
Update documentation on visualization to mention using pandas objects directly with matplotlib and the automatically installed date and time formatter.
Merge pull request #6660 from agijsberts/docs-plot-with-mpl

DOC: Mention date and time formatting available throughout MPL
API: add is_beg_month/quarter/year, is_end_month/quarter/year accessors (#4565)