Added if statement to GPMain to determine whether mutation, crossover,
or copying takes place. This may be moved somewhere else eventually, but
for the time being I think it makes sense to put this here.

Note: A small error fix in Mutation means Mutation Test now passes, but
for the wrong reasons. Currently, Mutation basically just copies to the
parent, so while we are getting a gptree of the correct length, it is
just a copy of the parent.