Use more custom types

[giordano]

More custom types should be defined and used throughout the package, in order to make it more Julian.

For example, I'd like to use SkyCoords.jl for sky coordinates, but I'd like it first supports entering coordinates with different units, see https://github.com/kbarbary/SkyCoords.jl/issues/6.

[kbarbary]

Out of curiousity, can you give an example of how SkyCoords types with unit parameters would be used in AstroLib?

[giordano]

For example to completely replace gcirc. I don't really like it as it is now (but it closely follows IDL AstroLib's definition). SkyCoords already has the method to compute the distance between coordinates, but lacks the possibility to input coordinates with different units.