Rolling reference comparisons

[sbrugman]

A wide variety of references is provided by popmon out-of-the-box. A reference may be static (a fixed training set, or the current dataset itself for exploratory data analysis) or dynamic (sliding or growing as more data becomes available). The reference is compared against batches, and they can be sequential (batched) or sliding (rolling).

Popmon should enable all combinations, and currently lacks external reference + rolling comparison.

	Reference	Compare to	Implemented
Self-reference	Static	Self (batched)	✓
External reference	Static	Batched	✓
Rolling reference	Rolling	Rolling/sliding	✓
Expanding reference	Expanding	Rolling/sliding	✓
External reference	Static	Rolling/sliding	✗

Thanks to @LorenaPoenaru!