Add functionality to automatically mark a Process class as an invalid cache source if it contains a non_db port

[sphuber]

The non_db keyword allows to define an InputPort in a process specification whose value will not be stored permanently in the database, but will just be available to the process itself during its lifetime. This probably means that reproducibility is violated and so instances of the process should not be valid cache sources. A Process class should automatically mark its nodes as invalid caches as soon as the spec contains a non_db port. We might need an override that allows a user to disable this behavior if they are convinced that the non_db port does not affect reproducibility.

Feature was requested by @giovannipizzi in a discussion during #5801