Document common groups of classes and make a view for them

[jeancochrane]

It would be nice to have a canonical definition of class groupings like "vacant", "improved", "single-family", "multifamily", etc defined in a table/view that we could use for reference and for filtering queries that operate on these class definitions. Two design options for implementing this would be:

1. Add a new seed to the ccao schema with two columns, group (string) and classes (array of strings)
2. Update the existing ccao.class_dict seed with a new column groups that is an array of strings, then define a view on top of this seed that expands out groups into the same schema proposed in bullet 1 above

2 is preferable to 1 since it retains ccao.class_dict as the single source of truth for class metadata, but 1 is an option if 2 is not feasible for some reason.

Adapted from this comment:

We should really have a table that defines various class groupings like this (and for stuff like "all SF", "all MF", etc.). Maybe we can add it to ccao.class_dict.