Fix regression in interact! behavior

See merge request !91

interact! was being called twice on each pair of layers due to a flawed recent change to the logic of how coupled processes work.

This flaw has now been resolved by (1) reverting back to previous "star" pattern implementation of interact! for joint coupled processes, and (2) allowing StratComponent to take non-coupled (single) layer types.

The possible drawback is that interactions between layers with multiple/coupled processes are now less automatic and must generally now be specified manually (though this is probably in general better for avoiding unintended behavior).

Preparatory changes for adding water processes

See merge request !90

Add simple bulk snow scheme

See merge request !89