Adds two functions. One in Chromosomes.py to determine the parental percentage of the chromosome. One in
Individual.py to determine the total parental percentage of the diploid genome.
Adds two classes to the CrossSim project. A BasicCross class that is able to cross selectively for a certain portion
of a chromosome to remain one parent and a WormIndividual that subclasses off Individual and places worm restrctions on mating, sex and number of chromosomes.

TODO(zifanxiang): confirm the subclassing hierarchy.