Add eval for correct_forking_fraction

[valedan]

What percentage of the time does the model take the correct path at a fork. Some complexity here - we are uncertain how best to deal with forks occuring after the model has already deviated from the solution. There's a version of this in alex's experiments folder to use as inspiration.

[traeuker]

@afspies you want to simply add your jazz somewhere or should we descope this?