make EPU offset for calibration an argument for `pol_C()` so we can use it better

[ambarb]

https://github.com/NSLS-II-CSX/profile_collection/blob/500232fad331b794f8a9b4beeb18492e4a0bfe2d/startup/99-helper_functions.py#L55

attn @cmazzoli just a heads up.