u/jbosch/DM-672

This is a small change that's just a switch from an older PSF-shapelet-fitting interface in meas_extensions_multiShapelet to a new one in meas_multifit.  I also slightly modified that latter interface, to make it so a function argument default could be set via config, because it was clear in the places I'm now using it that's where the value should come from.

Only complication here is that the diff is relative to u/jbosch/DM-641, not master (but this pull request reflects that).