reimplement shapelet PSF approximations

The CModel code we want to transfer from HSC in DM-240 currently relies on the old "multishapelet.psf" algorithm in meas_extensions_multiShapelet. That means we either need to convert that PSF-to-shapelets code in meas_extensions_multiShapelet to use the meas_base interface, or we need add new PSF-to-shapelets code in meas_multifit.

I think the latter is the better choice, even if we delay DM-240 as a result; the heavy algorithmic code is already available as primitives in meas_multifit, so it should just be a matter of packing those into a simple driver, creating a config class for it, and testing it on a few real and simulated PSFs to learn reasonable defaults for the configs.