These functions reshape matrices.
Vec() sends a matrix to a vector. The
columns of the vector are stacked into a rcx1 vector.
Vecdiag() extracts the principal
diagonal of a square matrix into a vector.
Diag() sends a vector into the principal
diagonal of a square zero matrix. It also zeros the off-diagonal
elements if the input matrix is square.
Shape() takes an rxc matrix to a new matrix
with nrows rows. The value of nrows must divide rc without a
remainder or the program will stop.