Parallel xrcmp and xrnan

[jmccreight]

This supports https://github.com/NCAR/wrf_hydro_nwm_public/pull/362

1. Made xrnan variable-wise and parallelizable. (Noted that crazy behavoir happens if you pass a dataset to multiprocessing or if you even leave a dataset open which a multiprocess also opens).

2. nccmp always for metadata diffs

3. allow core counts (>1) requested to trigger xrcmp instead of nccmp

4. I seem to have lost some minor logging changes I made to xrcmp.

[pep8speaks]

Hello @jmccreight! Thanks for updating this PR.

• In the file wrfhydropy/core/outputdiffs.py:

Line 291:13: E722 do not use bare 'except' Line 374:13: E722 do not use bare 'except'

Comment last updated at 2019-09-09 05:55:44 UTC

[coveralls]

Pull Request Test Coverage Report for Build 444

• 31 of 37 (83.78%) changed or added relevant lines in 6 files are covered.
• 10 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.4%) to 88.27%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
wrfhydropy/util/xrnan.py	19	25	76.0%
<!--	Total:	31	37	83.78%	-->

Files with Coverage Reduction	New Missed Lines	%
wrfhydropy/util/xrcmp.py	2	70.77%
wrfhydropy/core/outputdiffs.py	8	74.66%
<!--	Total:	10		-->

Totals
Change from base Build 441:	-0.4%
Covered Lines:	3138
Relevant Lines:	3555

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💛 - Coveralls