Added initial files for representation classes
Implement BaseRepresentation and 3 inherited Representations.

Implemented CartesianRepresentation, SphericalRepresentation and CylindricalRepresentation.
Implemented transformations between the three Representations by to_cartesian and from_cartesian methods.

Contains contributions by @astrofrog.
Added example of creating a Distance object from a Distance object.
Added initial tests for SphericalRepresentation
Added tests for SphericalRepresentation
Added tests for CartesianRepresentation
Added test for cartesian to/from spherical transformation
Added missing @classmethod
Fixed cartesian to spherical conversion
Fixed failing tests
Fixed copy= tests
Fixed typos
Added future imports and explanation at the top regarding the transformations.
Added tests for cylindrical conversion, and fixed failures.
Added PhysicsSphericalRepresentation
Added __all__
Add CartesianRepresentation.xyz property
Improved docstrings
Added API improvement suggestions
Convert distance to quantity just once