Added tests for materials.py\: RefractiveIndex class constructors, for `n0_const`, `n0_poly`, `n0_smcoeffs`, `n0_func`.
Added tests for materials.py\: RefractiveIndex class constructors, for `n0_const`, `n0_poly`, `n0_smcoeffs`, `n0_func`.

DanHickstein/EMpy,lbolla/EMpy,demisjohn/EMpy,DavidRimel/EMpy