This patch extends the matrix spec to allow matrix-by-scalar division.
Originally support for / was left out to avoid ambiguity for the
matrix-matrix version of /, which could either be elementwise or
specified as matrix multiplication M1 * (1/M2).
For the matrix-scalar version, no ambiguity exists; * is also
an elementwise operation in that case. Matrix-by-scalar division
is commonly supported by systems including Matlab, Mathematica