User-friendly interface(s) to get specific fit from demfit[] array

[AndrewEdmonds11]

At the moment, the track fit is sampled at various points (e.g. entrance, middle, and exit of the tracker for conversion electrons; inner and outer surface of the tracker for straight-line cosmics). These are stored in a vector (of vectors): demfit where the sample point determined by the surfaceId. This extra dimension does make e.g. plotting difficult but the extra dimension is required because we will want to add sample points in the future (e.g. back to the stopping target)

We need a nice interface for both ROOT and python such that an analyzer can get the fits for a single position. Something like:

df["dem_ent_fit"] = get_trk_entrance_fits(df["demfit"])