(first development by Aude)

the gravity potential files are now read using the same mechanism as the Earth Orientation Parameters, the UTC-TAI history and the JPL ephemerides files

added a feature allowing all tests to clear the already built reference objects (frames, time scales, solar system bodies ...) between each tests, thus removing the need to launch tests in separate JVMS. This allows to launch all tests directly from eclipse, and this speed ups maven tests by a factor 4 at least