AnySpace() Means two things: any space and space not yet defined

[dlfivefifty]

Some operators have definitions for any spaces, e.g. ConstantOperator is always diagonal. These have range/domain space as AnySpace. But other operators are not defined until spaces are attached, e.g. Derivative. At the moment they default to returning AnySpace (until a space is inferred by say additition), but really they should be returning something like UnknownSpace.