Rename ZGainScore to Velocity

[contsili]

The reason is that ZGainScore is not only a score but also does more things that are part of the general concept of Velocity models (eg thrivelines, conditional forecasting).

similarly rename zgain_score.py -> velocity.py and velocity.py (from the math_functions folder) -> velocity_helpers.py

[mglushanina]

I would also then rename the .score method to zgain_score or smth like that

[contsili]

I would not rename .score as the abstract base class (LongitudinalScore)defines it, so Zdiff, Zgain must all keep that exact name

[mglushanina]

ah, okay, fair
i would maybe then do a print saying something like "Computing ZScore", otherwise the user won't see at any moment which metric they use

Or we could just specify it in the documentation

[contsili]

Hmm, I think the only way is as you say specifying it with logs, in the docstrings and documentation.

Only way i see to keep the zgain name in the code would be to not rename ZGainScore to Velocity