Add support for per-layer callbacks

Adds support for registering callbacks from specified stratigraphy components. Callbacks should be implemented as struct types <: CryoGrid.Callback and registered via a callbacks(::Layer, ::Process) implementation. Corresponding implementations for criterion(::Callback, ::Layer, ::Process, state) (the callback condition) and affect!(::Callback, ::Layer, ::Process, state) should then be provided.

This pull request also includes some other minor bug fixes and maintenance changes, as well as a (hopefully final) renaming of CompoundProcess to CoupledProcesses which should not affect user code.

assigned to @bgroenke

approved this merge request

Tests pass locally.

merged

mentioned in commit 5abc253d