Revert #311 and add keywords

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• HISTORY.rst has been updated

After merging to master and before closing the branch:

• bumpversion (minor / major / patch) has been called on master branch
• Tags have been pushed

• What kind of change does this PR introduce?

Reverts #311 as per conclusions of Ouranosinc/xclim-benchmark#1 and pydata/xarray#3161. All calls to _rolling(da, dim='dim', window=window, mode=mode) have been turned into calls to da.rolling(dim=window).mode(allow_lazy=True, skipna=False).

• Does this PR introduce a breaking change?

No.

• Other information:

When used with xarray 0.15, a bunch of deprecation warning will appear from the explicit allow_lazy=True.

This PR fixes #316.

[coveralls]

Pull Request Test Coverage Report for Build 1430

• 8 of 8 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 89.528%

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Totals
Change from base Build 1428:	-0.2%
Covered Lines:	3454
Relevant Lines:	3858

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💛 - Coveralls

[tlogan2000]

Nice job on the benchmarking