cardGroup(A[,n]); A is a matrix with integral coefficients.
Return:
It returns a bigint. If we denote by ZA the group generated
by the columns of the matrix A, then it returns the number of
elements of the group of Z^m / ZA, where m = number of rows of A.
If a second parameter n is introduced, it will
only consider the first n columns of A. It returns 0 if Z^m / ZA
is infinite; this is, when rank ZA < m.