Should coordinate system subtypes be defined?

[desruisseaux]

Issues #4, #8, #13, #33, #42 and #46 have the same root and could be merged in a single issue here: the subtype element in PROJJSON does not refer to a class type. This is probably the reason why its name is not type like in other elements. Instead, subtype refers to an enumeration value similar to the value of the CS["foo"] element in WKT. That is why there is no CoordinateSystem subtype, and why all coordinate_system elements in PROJJSON refers to "#/definitions/coordinate_system" instead of a more specific subtype.

So a question is: should it be converted to the class hierarchy as defined in ISO 19111, and as used everywhere else in PROJJSON (e.g. for datum)?