Use automatic differentiation FParser subclass in ParsedFunction

This is a **work in progress**. I'd like for the ```ParsedFunction``` objects to use the auto diff FParser subclass (instead of the plain vanilla function parser). 

Ultimately I'd like to provide autodiffed gradients for ParsedFunctions (and maybe time derivatives as well). Gradients in particular would be of interest to MOOSE for example (e.g. in the context of parsed function initial conditions).

The approach I'd prefer is to _always_ generate those derivatives. There would be a tiny overhead when the function object is constructed plus a slightly larger memory requirement (for storing the additional functions for the derivatives). On the up side this would enrich the ParsedFunction object and provide the new capability in a straight forward way.