Function: matsupplement
Section: linear_algebra
C-Name: suppl
Prototype: G
Help: matsupplement(x): supplement the columns of the matrix x to an
 invertible matrix.
Doc: assuming that the columns of the matrix $x$
 are linearly independent (if they are not, an error message is issued), finds
 a square invertible matrix whose first columns are the columns of $x$,
 i.e.~supplement the columns of $x$ to a basis of the whole space.
