Fixes in subset, winter_rain_ratio, percentile_doy and heat_wave_total_length

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes issues discussed internally and brought up by external users on a different platform.
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
• bumpversion (minor / major / patch) has been called on this branch
• Tags have been pushed (git push --tags)

• What kind of change does this PR introduce?

1. This fixes a bug in subset.wrap_lons_and_split_at_greenwich caused by a typo, that made the tests core dump.
2. Also, a few fixes for bugs issued by external members:

• utils.percentile_doy used a panda-like indexing technique that failed with multidimensionnal datasets (not timeseries)
• Similarly, indices.winter_rain_ratio would also fail with multidim inputs. Fixed it.
• Finally, heat_wave_total_length was coded in indices and atmos, but wasn't included in the __all__ making it invisible. And there were no tests?? I don't know how that slipped away for so long.

• Does this PR introduce a breaking change?

Only unbreaking-changes.

[coveralls]

Pull Request Test Coverage Report for Build 1592

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.043%

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Totals
Change from base Build 1586:	0.0%
Covered Lines:	1988
Relevant Lines:	2258

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💛 - Coveralls

[davidcaron]

FYI, I ran into the percentile_doy bug in finch, I will base finch on xclim's master for my next tests and probably wait for xclim v0.14 for the next release.