Create a new input file section for nonlinear parameters

[hugary1995]

I am tempted to add a new input file section called something like [NonlinearParameters]. The advantages are:

1. Getting rid of the conceptual clash between Model and NonlinearParameter even though the latter derives from the former.
2. Easier to detect if an object is a nonlinear parameter, so that we can restrict certain API usage regarding nonlinear parameter registration.
3. Clearer documentation, as in the syntax documentation pages for [Models] and [NonlinearParameters] will be separated.

I'd explore this at some point.