Add a class which filters logged data

[OscartGiles]

We have the following classes:

• Monitor: Logs data and saves to datastore
• Registry: Loads all the logged data and then passes to HypothesisTest which applies drift metrics.

It would be good to have a new class that sits in the middle of Registry and HypothesisTest which allows you to query and filter data in the Registry:

Examples might include:

• Filter by the date the data was logged
• Filter by the feature value

You may also want to do split-apply-combine operations, for example:

• Split the data into weeks and compare each week to the reference dataset.

[sgreenbury]

Consider maintaining two copies of the dataset inside Monitor:

1. loaded datasets from backend
2. filtered datasets

For large datasets this will reduce the read time upon multiple uses of different filters.