revisiting snapshots at simulation stages

[feefladder]

problem

As mentioned in #164 and #179, the saving of snapshots can be revisited now we have the ability to update a variable inside a process step (#178 and #179). This would make using finalize_step in a model obsolete. However, this is quite a breaking change, and therefore should have a deprecation cycle.

Also, I don't know what can be done before finalize_step is removed.

proposal

1. issue a warning when the user uses finalize_step in a model, pointing to the documentation #182. and mentioning when the feature is to be deprecated.
2. actually deprecate the feature, and reduce the entire snapshots system