Implement map merging

[delucchi-cmu]

See #486

Implement mechanism to join a point-source catalog (e.g. object catalog) to a continuous map type.

Implement as a new operation, merge_map (name is negotiable) that, similar to a cross-match, allows for differing algorithms to suit the very different needs of map varieties. e.g.

• Photo-D style priors: would also want the custom function to perform some map_partitions style operations to reduce the data before leaving the routine.
• Pixel Mask: effectively filters out values in the point-source catalog
• Extinction map: adds a single (or a couple, e.g. value and uncertainty) floating point values per row in point-source catalog.