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
also it translates better to C++ recommendation of iterating over iterators like :+1: 
```c++
for(auto i = v.begin(); i != v.end(); ++i) // here you almost aways want to use !=, not <
```