[WIP] Simplify PhaseSpacePosition and Orbit classes

[adrn]

This makes use of the representation differential classes in astropy/astropy#5871 to clean up a lot of the code. A natural byproduct of this is that CartesianPhaseSpacePosition and CartesianOrbit are no longer needed, since there is now a unified interface to any representations and their respective differentials.

TODO:

• support <3D positions so nonlinear integrations work
• remove the velocity_coord_transforms.py and use the Differential classes instead
• figure out how to handle velocity frame transforms with the Differential classes
• update documentation and docstrings
• clean up all documentation that mentions Orbit or PhaseSpacePosition, check repr's (especially the orbits-in-detail.rst file
• make sure all mention of Cartesian* is gone
• make sure all code and doc tests run and don't use the old Cartesian* classes

API-breaking changes:

• Velocity frame transforms now return Differential classes
• Velocity coord transforms are gone
• CartesianPhaseSpacePosition and CartesianOrbit are deprecated

[coveralls]

Coverage decreased (-2.9%) to 82.214% when pulling a52c76b on dynamics/simplify-orbit-psp into 2c65d9e on master.

[coveralls]

Coverage increased (+2.0%) to 87.123% when pulling 9360f3a on dynamics/simplify-orbit-psp into 2c65d9e on master.

[coveralls]

Coverage increased (+2.0%) to 87.118% when pulling 7a9cc60 on dynamics/simplify-orbit-psp into 2c65d9e on master.

[coveralls]

Coverage increased (+2.008%) to 87.157% when pulling 220ffb9 on dynamics/simplify-orbit-psp into 2c65d9e on master.

[coveralls]

Coverage increased (+2.008%) to 87.157% when pulling a0d2e7e on dynamics/simplify-orbit-psp into 2c65d9e on master.

[coveralls]

Coverage increased (+1.9%) to 87.158% when pulling 83c65d6 on dynamics/simplify-orbit-psp into cc2bdeb on master.

[adrn]

TODO:

• Fix to_coord_frame and tests.
• Why is the analytic Isochrone action angle transformation failing now?
• G changed in astropy.constants - leading to a lot of test failures?
• Anywhere we accept v_circ and v_lsr, should just take galactocentric frame

[adrn]

• PhaseSpacePosition object should gain attributes of the components of the representation/differential(s). Maybe need to define my own mappings ('v_x' = 'd_x', 'v_xyz' = 'd_xyz', etc.)

[adrn]

• Still needs a bunch of tests for the new component name mappings
• Update documentation to point coordinate page to astropy.coordinates new features
• Audit docs for d_x or mistakes about not including cos(lat) term in proper motions
• PhaseSpacePosition should take a representation w/ differential attached

[adrn]

• Finish the example on integrating orbits in Barred galaxy-like potential (plot corotation, etc.) and add example of CRTBP (or make issue to do this)
• Support differentials in ND... representation/differential class (and add tests doofus)

[coveralls]

Changes Unknown when pulling 576c5ba on dynamics/simplify-orbit-psp into ** on master**.