Create meas_base Plugins for single-frame and forced measurement that uses the model-fitting primitives in meas_multifit to implement SDSS-style CModel magnitudes, in which we fit an exp and dev model separately and then fit the linear combination with ellipse parameters held fixed.

An old-style plugin has already been implemented on the HSC fork, and should be used as a guide; this issue involves adapting that implementation to meas_base and potentially cleaning it up a bit. Note that the HSC implementation cannot be transferred directly to the LSST side because the meas_algorithms APIs are slightly different on the two forks.