Need "mirror" for geom spec

[tclune]

Collection in History3g and ExtData3G need to be able to defer their geometry to their connections back into the model. In History, we've traditionally called this "native", though this is ambiguous for coupled ATM+OCN, and will be more so when we refactor SURF to use locstreams or xgrids.

Most of the necessary functionality is in place. The main decision point is whether to use an unallocated geom to indicate "mirror" or to create a new subclass of GeomSpec that is specific to mirroring. The former is probably less work, but the latter is more robust (I think).

@darianboggs @bena-nasa