Should Process classes be allowed to contain references to ORM entities?

[ltalirz]

At the moment, Process classes, such as work chains can contain references to ORM entities, e.g. as default values in the spec:
https://github.com/aiidateam/aiida_core/blob/b22f55155e7a602ac42b00edf62e57e4a458f55b/aiida/backends/tests/work/test_work_chain.py#L94

One problem with this is that it prevents creating a "clean slate" for tests:

1. Test A imports and runs workchain X (running may not even be necessary).
2. Database is reset as usual.
3. Test B imports workchain X and tries to run it. It fails because the default values point to database entries that no longer exist.

As noticed in #2421 this can be hard to debug, since test A and B can be otherwise completely unrelated.

One suggestion by @sphuber is to switch to lambda functions for defaults of workchains