DM-1223: new plugin for PSF shapelet approximation

Jim Bosch writes (at https://jira.lsstcorp.org/browse/DM-1223):
> There are also some small changes in meas_base (u/jbosch/DM-1223) that aren't really directly related, except that they were problems I had to fix in order to get the new plugins (and particularly their tests) working in meas_multifit.