Added
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- SoI : Possibility to force the frame of SoI propagator
- Date are now hashable and directly usable for plotting in matplotlib
- Propagators' iter() method accept Date iterable (allows to iter over variable step-sizes)
- Basic handling of covariance matrices
- Possibility to bypass the warning when creating a frame with a name already taken
- Tests on documentation examples

Modified
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- Kepler Propagator : Refactoring to allow any kind of numerical propagator
- Tle ndot and ndotdot parsing correction
- Change of default EOP policy to 'pass'. No more warning about missing data