Added developer documentation for optional dependencies
Fixed typo
Removed custom ImportError for most optional dependencies
Implement suggestion by @embray
Proof of concept for dynamic preferred representation
Allow attribute access for preferred representation attributes
Make preferred attributes dynamic by not creating properties in MetaFrame
Make sky coord units handling somewhat less spherical-centric
Add _attr_classes to Representations to indicate class types
More units generalization
First cut at generalizing SkyCoord inputs for other representations
Allow for initialization from non-default repr coord or frame instance
Generalize positional args
Fix test regressions
Add UnitSphericalRepresentation to _representations
Generalize input as BaseRepresentation case
Change from preferred_representation to representation
Make representation._attr_classes be OrderedDict
Add more concise way of defined represenation attr names and units
Fix SkyCoord and tests to drop preferred_representation