Added an Evolver as in input in Mutation class. This allows us to set up
an Evolver with the same parameters as the original GpTree (or different
parameters, if so desired) before running it through Mutation. To
Kirbie, this makes sense at the moment, but we may wish to change it
later. Currently, however, it appears to perform the desired behavior,
and all tests currently pass.

Also, removed popSize field from Evolver. Currently, Evolver is largely
just a container for various properties of a GpTree that must be
accessed by Mutation, and popSize is not needed here. Again, this may
eventually change.