Subspace class docstring.
Created and tested is_descendant method.

Created on BaseSpace, since it will apply to all space types (including bound
spaces later).  Special override for GlobalSpace, of which all spaces are
descended.  Note that GlobalSpace uses equality to check for "descendance";
therefore, all instances of GlobalSpace will be totally equivalent.  Only one
instance is ever really needed, but this just makes it a little safer.
Solve the trivial case of containing.

Harnesses the is_descendant method to check for the most trivial case of a
subspace being within another subspace.  Raises NotImplementedError for any
case more troublesome than that.