New object-based fitting framework

Should run a fit as

Fit this theory to this data, varying these parameters, subject to these constraints and using this algorithm (with this initial guess).

fit(theory, data, parameters, constraints, algorithm, initial_guess)

Too much fitting related information has crept into scatterers. This probably means we need another layer of objects to represent fitted objects. These will in turn generate scatterers.

Scatterers are supposed to be a purely geometric representation of scattering objects, and shouldn't know anything about how we use them in fits (people who are only doing scattering calculations should not need to know anything about the fitting stuff we do.