Spectral Flux report improvements

[moellep]

• Allow the SRW Spectral Flux report to run on NERSC.
• For idealized undulator, add the "Accurate" magnetic field treatment option (use the Approximate method by default for idealized simulations).
• Add Spectral Flux report with accurate/approximate methods to Arbitrary Source simulations

[moellep]

I found that the "Accurate" method could not be selected for idealized undulators (sm_mag=2): Exception: Accurate Magnetic Field is not defined

Also, with an arbitrary source, an approximate magnetic field could not be selected (sm_mag=1): Exception: Approximate Magnetic Field is not defined