Specify which media variables are time varying

[williambdean]

Allow users to decide which variables are time dependent or not. Basically, create the flexibility so that each user can decide which model parameter should have time variability.

once we merge? My thought is that you should be able to pass a bool or a list[str] (list of channel names) to time_varying_media, and the latter would set gp multiplier on each listed channel.

Originally posted by @ulfaslak in #752 (comment)

Following #752, this might be good direction to take TVPs

Related to #712

Would be interested to hear what other people think or if this would be helpful to others