DataSet.filters() / filter variable concept

[alextanski]

We would want to have the option to declare specific "filter" variables, which would simply have one category (1 = "active" or something similar). These would get the recoded_filter property: Currently, such variables are only generated by DataSet.from_batch(additions={'full', 'filters'}) (using the inner private _manifest_filters() function). It would be good if we could have a proper method that generates such a variable from an arbitrary complex logic so it could be part of a Batch definition's variable list / could be used by the Batch class vice-versa, maybe similar to Batch.add_filter('FILTER_VAR_NAME').

@AndersFreund: I am adding you as we talked about this regarding the YG / Crunch use case. Your ideas can be collected in this ticket.

[nitsrek]

done and merged