making basicSpace abstract

[Eric Dean Tribble <tribble>]

BasicSpace is a concrete class that holds onto a category that tells
it what it's position type is.  This makes CoordinateSpaces some kind
of confused combination of objects and types.  They quickly fail at
representing types, because they can't participate properly in type
distinction mechansims (the type-converters, CHOOSE, etc).

I propose we make BasicSpace an abstract class.  Each basic coordinate
space is a subclass of the abstract class.  This way we can properly
implement the routines which verify OrderSpecs, Regions, and the like.
The basic space can also properly act as a type (because it has one!).

Is there any reason not to do this?  I think this all works out fine
with Cross coordiante spaces, etc.

dean