They're not needed now that we're using intervals, which have their own
hull function.

The old method was unused now that everything's being changed to take
IntervalBoxes, so I decided this was more useful.  Now, it returns a
3-dimensional array obtained from calling the individual method on each
element of the vector, effectively concatenating the
automatically-vectorized version on the third dimension.

This order, with each column as a state vector, matches functions like
lsim from ControlSystems, and offers better cache behavior with Julia's
matrix ordering.  It also matches many uses of the function, which
immediately transposed its return value.