More builder refactoring

- Extended the idea of NeuronsBuildState to all objects that
  have build artifacts, and exposed them through
  the data dictionary (i.e., the probe system)
- Reverted builder to a callable class
Added test for Model save/load

This required implementing `__eq__` methods for all objects.
This has the side effect of requiring labels in `test_context` since
`in` apparently uses `__eq__` to determine membership in a list.
Okay, so maybe we need unique IDs.
Readded test_seeding

- In test_builder
- Looks at the built model's params