DM-191: Measure and save aperture corrections in SFM

https://jira.lsstcorp.org/browse/DM-191
Interesting.  I've subconsciously adopted the opposite convention, I think because I find putting the operator at the beginning of the line much easier to read (i.e. more like handwritten math) when there are multiple such operators, e.g.:
```
longVariableName += otherReallyLongVariable
   + anotherReallyLongVariable
   + yetAnotherReallyLongVariable;
```