How should physics scheme tendencies be accumulated over the timestep?

As pointed out by @gold2718 in pull request #7 , it is currently unclear how changes in, say, tracer quantities should be gathered or accumulated from each physics scheme and then passed on to the dycore. There are currently three options on the table (although feel free to add more):

- Keep the current scheme (i.e., only track updated quantities)

- Move to accumulating tendencies (for passing back to the dycore)

- Have CAM accumulate the type of tracer feedback required by the dycore (e.g., updated quantity for FV, tendency for SE).

A decision needs to be made at some point, but for now this issue purely exists as reminder to discuss the different possibilities. Thanks!