Support file_refit = "on_change" with brm_multiple

[martinmodrak]

For first implementation of #1088 only the simpler case for single models was implemented. Support for brm_multiple has a WIP code at https://github.com/martinmodrak/brms/tree/extended_cache_brm_multiple but incorporation awaits better support for testing if it works.

[paul-buerkner]

We should also consider #997 in the process as we will need to store all data.frames in the brm_multiple object.

[krassowski]

Can I help testing it? This would save me hours of re-running the models. It feels like on_change is especially beneficial with MI.

[paul-buerkner]

I will let you know once there is an implementation to test.