Change default flag for open_mfdataset. Fixes Issue #2064

[bonnland]

• Closes concat_dim getting added to *all* variables of multifile datasets #2064
• Tests added
• Fully documented, including whats-new.rst for all changes and api.rst for new API

I'm not sure I should add a test, because it's just changing the default behavior, and it requires a dataset file to be added.

I'm not sure it needs additional documentation, because Sphinx should update the default flag. Should what-new.rst be updated?

[codecov]

Codecov Report

Merging #3120 into master will decrease coverage by 0.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3120      +/-   ##
==========================================
- Coverage      96%   95.72%   -0.28%     
==========================================
  Files          63       63              
  Lines       12777    12777              
==========================================
- Hits        12266    12231      -35     
- Misses        511      546      +35

[bonnland]

I'm seeing that five unit tests are failing on some of the builds. I'm looking into addressing the failing tests now.

[jhamman]

Closing this as stale. Thanks @bonnland for the contribution. Feel free to reopen if there is interest in picking this up again.