New Type for Utility

[mileslucas]

I'm curious of the thoughts of creating a type along the lines of CartCoords, UnitCoords, CartesianCoords, etc. that serves as the unit-sphere implementation of the coordinates.

This is already used in the code a bit, via coords2cart and cart2coords. My theory is that we can create some <:AbstractSkyCoords type that serves as the base, default representation of all SkyCoords. this means as long as we provide a conversion to this type for each new type, we could allow efficient code reuse using a pretty Julian style of generics with fallbacks and separate optimized code.

This, itself, was part of my features list #30 but i'm starting to see how we can use it to better serve our package in terms of functionality, not just extra features.

[helgee]

For reference: https://github.com/JuliaAstro/AstroBase.jl/blob/master/src/coords/Coords.jl