Add Doxygen docs for PTI and include them in the general Doxygen build

[SigmaX]

To use the parameter-tuning interface, users have to make use of 3 or 4 C++ classes, all of which are undocumented.

There is also some (outdated) Javadoc buried in there somewhere for the Java side of the PTI framework. It's only relevant to people who are writing their own ECJ extensions (the User Guide chapter on ECJ covers all users really need to know), but, for the sake of completeness, there might be a way to merge that documentation with our doxygen build if we felt like it.