U/jbosch/dm 1223

Jim Bosch writes (at https://jira.lsstcorp.org/browse/DM-1223):
> This is a plugin interface (in the new meas_base framework) for my new shapelet-PSF-approximation code. The deep algorithmic code this delegates to has already been reviewed, so this is just the Python layer that lets us run it as a plugin. Most changes are in meas_multifit, on branch u/jbosch/DM-1223.