Standard gamma-ray calibration lines

[markbandstra]

It might be helpful to provide some way besides using NNDC to quickly access the line energies of standard gamma-ray sources, e.g., Cs-137, Co-60. And to tie this into the energy calibration somehow, e.g.,

Calibration.fit_sources(["Cs-137", "Co-60"])

We would have to think about how to handle sources with many lines like Eu-152, maybe by requiring a minimum number of them to be used.

[markbandstra]

@heller3