Reverted deprecation of `~.dates.num2epoch` and `~.dates.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 and fixes those functions to work with
`~.dates.get_epoch`. 
