Enrich the variable catalog generated by CrunchCube objects

[jonkeane]

variables() on a CrunchCube return a variable catalog with variable tuples which don't have all the same methods for accessing subvariables.

We should work on making methods like subvariables(), names(), aliases() etc. work on variable tuples (when that information is available, eg variable tuples form CrunchCubes had subreferences with names and aliases, however variable tuples from the variable catalog doesn't).

• names() for subvariables/subreferneces from a CrunchCube
• aliases() for subvariables/subreferneces from a CrunchCube