fixes Issue #14 re start pattern

A number of element patterns were added to the Node.choice and edge.choice patterns.
The root element is, at this point, allowed to be any of thee elements.
Also corrects some errors regarding an arg edge (line 955), and
fixes the content model of IntervalAlgebraOperator (line 1054)

Signed-off-by: greenTara <taraathan@gmail.com>