DM-191: Measure and save aperture corrections in SFM

https://jira.lsstcorp.org/browse/DM-191
This is defined as a multiplicative correction (i.e. it's in flux units, not magnitude units).  So a negative correction actually makes your fluxes negative.  I suspect this condition will never actually happen, and so it's not worth the attention of a log or an extra flag field; I just wanted to make sure that we didn't put any NaNs anywhere with setting the flag.