feature/declarative deterministic

[rhayes777]

illustration of creating declarative, deterministic graph using compound priors

Also adds syntactic sugar where you can do this:

import autofit as af


model = af.Model(af.Gaussian)

print(model.fwhm)

As af.Gaussian.fwhm is a property that is a function of the arguments to the class, model.fwhm is now a compound prior