Turn off "Compiling additional model methods..." message when calling `init_model_methods`

[ben18785]

Is your feature request related to a problem? Please describe.
I am creating a package that repeatedly calls init_model_methods (each time I use a different dataset with my Stan model) to give access to log_prob. Each time it does so, it outputs

Compiling additional model methods...

to the console.

Describe the solution you'd like
I'd suggest either removing this informative message or allowing users to do so.

Describe alternatives you've considered
I could capture the output in my package and then remove it from users' view, but that feels messy.

[jgabry]

Maybe this can be handled as part of #894