Use TrajectoryData in converter tests

[blairlyons]

What needs to happen?

Currently most converter tests call JsonWriter.format_trajectory_data(trajectory_data) and then verify the data in dict format. Ideally we want the converter tests to not involve JsonWriter, so we could instead:

• in converter tests, test that the TrajectoryData object is correct while still a custom Python object (i.e. just use trajectory_data rather than first converting it to dict with JsonWriter.format_trajectory_data(trajectory_data)
• make separate tests for all the Writers' format_trajectory_data(...) functions

Why should we do this?

So tests are more modular and different objects are tested separately

When does this need to get done?

2023