Unit tests

[arturtoshev]

Addition of a few new unit tests:

• case_setup including preprocessing (allocate / update), neighbors search, and integration loop
• push-forward utils
• rollout loop. Defines a "cheating" model, which returns the target accelerations, and results in perfectly reproducing the trajectory up to numerical precision during the integration. A new dummy dataset of 3 Lennard-Jones particles (<1MB) is added to this unit test.

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