add checkDependency for student snopt

Resolves #415 
1. add 'studentSnopt' in `checkDependency`
2. `NonlinearProgram` set solver to 'default', which checks snopt, studentSnopt,fmincon,ipopt in sequence.
i think this should be `case {'snopt','studentsnopt'}` and you just set both if either is requested.  then zap the separate (but almost identical) 'studentsnopt' logic below.