Twobody

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- Removed `astropy-helpers` from repository.

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- Added the option to initialize a ``KeplerElements`` or ``KeplerOrbit`` object
with a specified velocity semi-amplitude, ``K``.

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- Bugfix release.

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- Added support for long-term polynomial velocity trends.

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- Added option to specify just radial velocity to ``Barycenter`` initializer.
- Docs improvements.

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- Initial release.