gateff is unnecessarily tabulated

[ameliajo]

In the gateff function, we generate effective temperatures for various materials, as processed by General Atomic's Reference Manual for ENDF Thermal Neutron Scattering Data by Koppel and Houston [https://www.osti.gov/servlets/purl/4075168]. It turns out these values are really polynomial, so it'd be nice to just replace them. However I don't want to do this because it definitely isn't that great of accuracy with 2-ish degree polynomials, so let's wait to actually use it until more of thermr is written so that I can understand its impact more. Code is already there, just not used.