CAMDEN should initialize tendencies

CAMDEN should initialize all tendencies in `physics_tend` to zero at the beginning of each physics timestep.
It would be nice if generate_registry could generate a routine to do this.

Start by locating the physics_tend initialization at the beginning of each physics timestep and the generate_registry entry point. Determine how all tendencies are represented and whether the generator should produce the zero-initialization routine; done means every tendency is zeroed at each timestep and the generated behavior is verified.

Written by the indexing model from the issue text.