Reconcile differences in two-photon continuum with IDL results

[wtbarnes]

The two-photon continuum calculation still shows non-trivial differences as compared to the equivalent calculation in IDL. See discussion in #297. Currently, the comparison tests are marked as "xfail" though across almost all wavelengths and temperatures, the differences are less than 10%. Some differences may be due to the lack of inclusion of the dielectronic transitions in the full calculation, but this needs further investigation.