Model accepts constraints. Added relevant examples. Other minor changes
The model function now accepts constraints as an optional argument. This seems to work quite well. A few relevant examples have been added. For some reason, this seems to work better if A^2/\sigma is used as the gfn prefactor, but this would break unconstrained fitting.
XPS_simple has been renamed to XPS_tutorial, because it is no longer simple. Later, I will make XPS_simple into an actually simple demonstration.
Also tweaked file import options to add verbose tag. It is verbose by default.
Normalization problem/question still needs to be solved.