Grouping map units by affinity

[davenquinn]

In prototype map filtering views, we can easily find units that truncate at jurisdiction boundaries based on arbitrary-ish mapping decisions. It would be ideal to have a system for grouping map units by affinity based on:

• lexicon hierarchy
• age
• lithology

and an API that makes the right decisions for flexible display in a large number of cases.

[davenquinn]

This will require a variety of backend fixes