Unit test for gradient calculation with satellite ephemeris

[rpoleski]

The test test_get_d_u_d_params() in test_FitData.py has to be corrected:

1. It uses private function (_get_d_u_d_params()) which should not happen.
2. It only tests if answer is different than for calculation without satellite ephemeris, but it does not check if result is consistent with independent calculation.