Fix Conditional Circular Import

I noticed that there are still some circular import errors between `trace` and `observation`, the strange thing being that if `trace` is being imported before `observation` then nothing happens but importing `observation` on its own causes a circular import.

My understanding is that this is because:

- `observation` imports `Observation` from `observation.py` which imports from `trace`, which runs the imports from `trace.py` which imports `Observation`. So `observation` has a circular import built in.
- `trace` imports from `trace.py` which imports `Observation`, which runs the imports for `observation.py` which imports `State` and `Action` but not `Trace`, so no direct circular import is caused.
- Importing `trace` before `observation` fixes the circular import as `trace.py` (and the second `Observation` by proxy) won't need to be imported again.

Now the issue around circular imports was addressed in #143 and #145, so not sure if this is by design or if this use case was missed (naturally anything trace-like was always imported before anything observation-like as they would be used first). At the very least the user should get a more helpful message so they're not left scratching their head at an internal import error should they happen to import anything from `observations` first.

Additionally (and perhaps this should be a separate issue) but while we're on the topic of observations vs. traces, `observations.print(view="details")` crashes as the data structure isn't totally set up the same as `TraceList`, so maybe `ObservationLists` just shouldn't inherit print functionality?

@e-cal I think you designed most of the `ObservationLists`/`TraceList` functionality so maybe I'm just missing something, would like to know your thoughts on these 😃