Add MatrixBuilder, a unified interface for shapelet matrix evaluation

MatrixBuilder provides the functionality previously present in
MultiShapeletMatrixBuilder, ModelBuilder, and some private code
in meas_multifit's psf.cc, with a unified interface, better code
reuse, and a design more amenable to future micro-optimization.
Remove old MultiShapeletMatrixBuilder

This has been replaced by MatrixBuilder
Remove old ModelBuilder

Replaced by MatrixBuilder
Rename MultiShapeletFunction's "elements" to "components"

This brings MultiShapeletFunction's nomenclature in line with that used by
MultiShapeletBasis and MatrixBuilder.