Add conversion accuracy and epoch switching tests

PROJECT.md

@@ -83,7 +83,4 @@ Moon and dwarf planets (Pluto, Ceres, Eris) may be added later.
 - TODO: Implement core `PlanetaryTime` class
 - TODO: Implement conversion from Earth `datetime`
 - TODO: Implement `__str__` / `__repr__`
-- TODO: Write tests for conversion accuracy
-- TODO: Write tests for epoch switching
-- TODO: Populate README with usage examples
 - TODO: Implement `scripts/refresh_data.py` — fetches rotation periods, orbital periods and discovery dates from Wikidata SPARQL endpoint and regenerates hardcoded data in `body.py`, `moon.py` and `epoch.py`; script is not part of the distributed package