MOSES: disc variables with n=2 mishandled

[linas]

Something inside of MOSES is mis-handling disc variables with an arity/mutiplicity of 2. I suspect that field_set is the code that's broken, but not sure.

The quickest demo of this is to run the nmax example program (learning/moses/example-progs/nmax.cc) and set n=2 as the input. This program works fine for n=3 and greater. For n=2, it should behave the same as onemax, instead it fails dramatically, finding an utterly incorrect solution.

I'm marking this as "high importance" because I'm assuming that discrete variables with n=2 might be be/become commonplace, and this would be very hard to debug or even notice, since, for more complex problems, the algo would simply fail to find a good solution, and the failure might not even be noticed.

Will need to create a unit test case for this too...