categorical data in Goose

[Jon Trowbridge <trow emccta com>]

So our discussion a few days ago about a generalized type system for
Goose led me to the conclusion, upon reflection, that I didn't
have a good handle on the problem.

To try to sort things out, I decided to implement a container for a
completely different type of beast, categorical data.  This code is
largely untested... but at least it compiles, and it shouldn't be
*too* broken.

The new code is in src/Category.{cpp,h} and src/CategoricalData.{cpp,h}.
I've also written a pseudo-tutorial type of thing on the scheme, which
is in doc/categories.txt.

Comments are welcome.  I leave the problem of how to integrate this
with the ASCII import stuff as an exercise for the reader. :-)