Multiply matrices MATRIX-1 and MATRIX-2, storing into MATRIX-3 if supplied.
If MATRIX-3 is not supplied, then a new (ART-Q type) array is returned, else
MATRIX-3 must have exactly the right dimensions for holding the result of the multiplication.
Both MATRIX-1 and MATRIX-2 must be either one- or two-diimensional.
The first dimension of MATRIX-2 must equal the second dimension of MATRIX-1, unless MATRIX-1
is one-dimensional, when the first dimensions must match (thus allowing multiplications of the
form VECTOR x MATRIX)