IndividualField: add operator== and operator!=

Add implementation of the two operators for both IndividualField and
IndividualFieldValue so we can write tests more easily and compare
objects. Comparison is based on value equality. The Genotype class
already had the operators implemented (thanks @kshakir)

fixes #187
!= is a cheaper operation in most objects (not a difference for ints...) and gives you the same behavior in these types of iterations. Same is true for ++i vs i++, albeit this one is faster even for ints.

This is in Bjarne's book. His recommendation.