using `num_shots` and not `reps` or `repetitions`

[petergthatsme]

@mghibaudi we were thinking if it could make sense to do a global replace of reps or repetitions for num_shots. It seems we're using all those in different places but it could make sense to make things more consistent (also num_shots seems to be used by other vendors)
thoughts? i can do a PR if you're ok with that.