API Changes for 3.3.1

Deprecations

Reverted deprecation of num2epoch and epoch2num

These two functions were deprecated in 3.3.0, and did not return
an accurate Matplotlib datenum relative to the new Matplotlib epoch
handling (`~.dates.get_epoch` and :rc:`date.epoch`).  This version
reverts the deprecation.

Functions ``epoch2num`` and ``dates.julian2num`` use ``date.epoch`` rcParam

Now epoch2num and (undocumented) julian2num return floating point days since ~.dates.get_epoch as set by :rc:date.epoch, instead of floating point days since the old epoch of "0000-12-31T00:00:00". If needed, you can translate from the new to old values as old = new + mdates.date2num(np.datetime64('0000-12-31'))