Think about whether to make the interior value optional in `TracePair`

[majosm]

In certain use cases, only the exterior value is needed from a TracePair. For example: in one possible implementation of a multi-volume simulation in MIRGE-Com, the simulation driver would call inter_volume_trace_pairs (from #236) outside of the subdomain operator calls, feed the exterior values from the result into a set of boundary condition constructors, and discard the interior values. These boundary conditions would then be applied inside the individual operator calls (and at that time they would then be supplied interior values by the operator).

This issue is a reminder to think about whether the interior value should be made optional in TracePair, so that inter_volume_trace_pairs, etc., can be configured to only return the exterior values, or whether it should be left alone and we should just depend on lazy to eliminate any performance penalty.

cc @inducer