Closes #313. Return success flag from GSI solver

[mgoodson-cvd]

• Adds a unit test for the Newton solver class
• Modifies the Newton solver class to return success or failure based on convergence instead of returning a redundant reference to the input
• Modifies the solveSurfaceBalance routines of the GSI solvers to return success or failure based on convergence

[capriatim]

I tested it. It works fine.

It is a minor change. The version should be updated to 1.3.0

I would add a function to get the convergence status to be included into the fortran wrapper without breaking backward compatibility.

[mgoodson-cvd]

@capriatim glad it works for you.

I didn't bump the version number because this doesn't technically break compatibility. It used to be void.

I see two potential ways to add this to the Fortran wrapper. The success flag isn't stored anywhere; it is returned from the solveSurfaceBalance() function call. The correct way to do this would be to make solveSurfaceBalance a function rather than a subroutine in the Fortran interface, but that would break backward compatibility. Alternately, I think I could add a success variable as an optional argument; not quite sure how that works with the implicit interfaces M++ currently uses for Fortran and maintaining backward compability. I can try it though.