Major update

* Put commands into separate file.
* Changes in type theory:
** Added the empty set as a basic type, changed product type semantics
to allows enums.
** Added definition for typed expressions.
** Added uncurried version to "function arrow" example.
** Desribed function application w.r.t. types.
** Cosmetic changes.
* Neural systems: cosmetic changes, harmonized formatting with the rest
of the document; put type notation back in.
* World frames:
** Updated frame properties; changed "minimally n-dimensional" to
"n-dimensionally embeddable".
** Added example of a 2-dimensionally embeddable world.
** Defined edge direction/position function for n-d.e. worlds.
** Began writing the standard semantics for jungle worlds.
* Began describing the agent structure and the agent interaction with
the world.