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.
you don't need this check.  and I worry that it won't actually work (since you have studentSnopt here, but set studentsnopt below).