Need to join/close processing pool

[ELind77]

The version of pyevolve available through pip doesn't join or close the processing pool in GPopulation.evaluate(). This results in an EXPLOSION of RAM usage, so much so that it becomes almost unusable when the multiprocessing flag is True.

The current code on the master branch appears to fix this issue, all that needs to be done is to update the code available in the repos. I think that this is a big enough issue that it merits a new patch.

-- Eric