Add new surface energy balance parameterizations

See merge request !34

Move freeze curve code to registered Julia package

See merge request !92

Types no longer have `Param` values by default. They must be added by the user or by the `parameterize` method which automatically wraps all numerical quantities with `Param`.

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