Fast Toeplitz band matrix multiplication

Fast and storage-efficient multiplication by Toeplitz band matrix, using Matlab's filter function.

Multiplies a Toeplitz band matrix TB with a matrix x ("Toeplitz" means:

constant along each diagonal, "band" means: only diagonals along a

bandwidth around main diagonal are non-zero) If transp='prime', then TB'*x

is calculated. If x is a multi-dimensionaly array, then multiplication

goes along dimension dim, i.e. the "n-mode product" of the tensor x by

the matrix TB is calculated (see de Lathauwer et all, SIAM 2000, for a

definition). Never sets up the entire Toeplitz matrix, which is

storage-efficient. Option 'fill' implements a modification of TB to make

row sums constant; if TB is a Markov transition matrix, this corresponds

to confining jumps to within the grid boundaries.