Check that requested core level is allowed by pseudopotentials

[jtv3]

In the OPF stage the code should make sure that the core level requested (for XAS, XES, RIXS) is actually a core level according to the pseudopotentials that are being used. For instance, many pseudos include semi-core states, which means if you requested the M edge of a 3d transition metal the code will crash in the SCREEN stage.