read_adf15

Do you want a `read_adf15` function? I understand that ColRadPy will calculate it from passed _adf04s_ but as people keep on rewriting `read_adf15` (including people that use ColRadPy) it would be useful to have this in one place. Feel free to use mine or use it as a starting place. I do use `xarray` so it would add a dependency if you used it as is but that is easy to change. https://github.com/dgahle/baysar/blob/feature/inter-baysar-cuda/adas/python/read_adf15.py