Save generation to resume from

[jauerb]

Several people have asked about this.

This will require:
(1) an option in the evolutionary conf to specify you want to save all individuals and not just the best
(2) the ability to initialize a new run with this saved population
(a) right now, we cannot go from .json format back to RobotRepresentation objects, so "unserialize" methods will be needed for that
(b) as a simple temporary solution I wrote up https://github.com/jauerb/robogen/blob/master/utils/json_converter.py which will allow converting from .json back to txt format which we can use to seed a new run