Track nDoF only returns the sum of calibrated size and not the actual DoFs.

I think that the computation of the DoF of a track is wrong.
At the moment it is returning just the sum of the calibrated size.
https://github.com/acts-project/acts/blob/main/Core/include/Acts/EventData/TrackHelpers.hpp#L46
I think shouldbe NDoF=(sum of calibrated size) - fit parameters.
I have a doubt about time. Do we count the time in the DoF or not if there are no time measurements?
I think it should be -5 for a 3D track and -6 for a 4D track (i.e. if there is at least one time measurement). If there are only 3D measurements than time is a function on the other 5 TPs, hence not a DoF. Can the TrackHelpers tool be improved to reflect this or is the idea to leave it to the clients?
@paulgessinger @andiwand