Incomplete spectral data sets for optical constants

[hannes-vernooij]

k values in the materials CsI and SnTe are incomplete. All spectral values are set to 0, this fails to accurately represent the optical properties of these materials.

[njroussel]

Hi @hannes-vernooij

I've missed this, sorry.

Unfortunately, it seems like the reference we have been using (www.luxpop.com) is no longer maintained. I'd welcome a PR with updated values for those materials and appropriate references for them.