DM-191: Measure and save aperture corrections in SFM

https://jira.lsstcorp.org/browse/DM-191
I understand the convenience, but I'm not sure it's a good idea --- this would allow a configuration to get out of sync with itself.  Though I guess we don't want a run bombing because we weren't able to load a pluggable algorithm.  And a warning may not be a good idea so far down in the loop over sources.  OK, I think you've convinced me...