Resampling indicator

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes ResamplingIndicator #867 and fixes Allowed input freq as a parameter #898
• 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?

• Split all resampling-related functionalities of the indicators into a new ResamplingIndicator subclass. I did this by creating private pre- and post-processing methods that are called in __call__. This makes it easy to "inject" functionalities in the indicator's processing. The "resampling-related functionalities" are : checking for the allowed "freq" arg and handling of missing values.
• Have the data input frequency expected by the indicator stored in a class attribute. This makes it easy to modify it when creating indicators. This improves the changes in Better check_freq, complete anuclim #885.

Does this PR introduce a breaking change?

I don't think so.

Other information:

I think that with this change it becomes a bit clearer what are the uses cases for conventional subclassing of the Indicator class:

1. Add functionality by overriding one of the pre- or post-process methods.
2. Convenient way of modifying the following attributes : context, src_freq, realm, title, abstract, keywords, references, notes and allowed_periods.
   In both cases, the goal is to create a new base class and then to create multiple indicators.

I'll put this in a "developer's note" section of the doc in a further PR.

This is also a first step towards a way to inject the indexer kwargs as planned in #899.

[aulemahal]

@bzah With this PR, I modified the way one can specifiy which input frequencies are accepted. See the data/anuclim.yml for an example. I hope it doesn't break your code too much!

The good part is that one can now inherit from an indicator created from Daily and still modifiy src_freq. Daily is only a convenience class, it now doesn't hardcode the frequency check, like it did before.

[coveralls]

Pull Request Test Coverage Report for Build 1481920829

• 106 of 110 (96.36%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 90.257%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
xclim/core/datachecks.py	1	2	50.0%
xclim/testing/tests/test_indicators.py	18	19	94.74%
xclim/core/indicator.py	61	63	96.83%

Totals
Change from base Build 1477070386:	0.01%
Covered Lines:	12654
Relevant Lines:	14020

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💛 - Coveralls

[bzah]

Thank you for your concern (and for the feature!) @aulemahal, it won't break my code I'm using atmos daily indicators directly and I have simply disabled the daily check for now on icc v5.
After this PR I might inherit them and change src_freq, but there is an ongoing discussion to see which indices/indicators should accept other frequencies.

[huard]

Good for me.