Revised for ESMPy version 620_01b.

Addresses
  * #685 'Upgrade regrid_conservative_via_esmpy to latest ESMPy version'
  * #686 'Explain how to make "regrid_conservative_via_esmpy" work.'

NOTE: The functions aren't tested in Travis, as it doesn't install ESMPy/ESMF.
Tested locally ok with ESMPY version version 620_01b.

Reviewer: cpelley

TODOs:
  - [x] move whatsnew item from 1.5 to 1.6 whatsnew list
  - [x] final rebase