Better check_freq, complete anuclim

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes anuclim add docs / refactor / add indicators (?) #538
• 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)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.
• bumpversion patch has been called on this branch
• The relevant author information has been added to .zenodo.json

What kind of change does this PR introduce?

• Extends check_freq so we can create Indicator classes accepting more than one input freqs.
• Creates a DailyWeeklyMonthly indicator subclass for use in the anuclim virtual modules.
• Adds many missing metadata for the anuclim indicators.

EDIT:

• When src_timestep is not given, it is inferred in Mssing classes.

Does this PR introduce a breaking change?

No.

Other information:

[coveralls]

Pull Request Test Coverage Report for Build 1399680918

• 48 of 50 (96.0%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 90.227%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
xclim/core/missing.py	2	3	66.67%
xclim/sdba/base.py	0	1	0.0%

Totals
Change from base Build 1399600752:	0.02%
Covered Lines:	12455
Relevant Lines:	13804

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💛 - Coveralls