Rename combine functions

[TomNicholas]

Finishes #2616 by renaming
combine_manual -> combine_nested,
combine_auto -> combine_by_coords,
and renaming the arguments to open_mfdataset:
combine='manual' -> combine='nested'
combine='auto' -> combine='by_coords'

[x] code changes
[x] docs changes
[x] what's new updated

[pep8speaks]

Hello @TomNicholas! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2019-06-26 09:12:59 UTC

[shoyer]

Thanks @TomNicholas, these new combine methods are long awaited!