FTI - Add multimesh_instances_reset_physics_interpolation()

[lawnjelly]

Convenience function to allow resetting all instances on a MultiMesh at once, rather than calling individually per instance.

Notes

• I had been meaning to add this function for a while, there had been several requests to make things easier for bulk resetting.
• The naming is slightly confusing, as we have MultiMeshInstance as a scene node, but the MultiMesh resource contains instances themselves (which are reset here), but that naming seems to have been set in stone historically and can't easily be changed now.
• An alternative might be to call the functions reset_all_instances... rather than reset_instances..., given that the current is very similar to reset_instance... used already, am happy to change if this is preferred.
• Also applicable in 3.x.

[rburing]

Code looks good and API is useful.