Preprocessed distinct grouping mode

[mathemancer]

Fixes #414
Fixes #411
Related to #429

This adds a preprocessor argument that can be given with the distinct grouping mode. When given, this argument defines a preprocessing function for each column before the distinct grouping is applied. In particular, this allows:

• grouping by email domain
• grouping by URI authority
• grouping by URI scheme

Technical details

This paradigm (i.e., preprocessing before applying a given grouping) can also result in other exotic grouping modes, for example grouping by email domain prefix.

The preprocessor list argument (preproc) must be a list of function id strings the same length as the given list of grouping columns. If no preprocessing is desired for a given column, that place in the list should be null.

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[mathemancer]

This is blocked by

#1312 and #1314 . It's marked as a draft until those are merged.