Generalized orthogonalization

Orthonormal basis of a set vectors with respect to a non-euclidian scalar product

Input:
A: (m x n) matrix
B: (m x m) matrix, symmetric positive-definite
Output:
Q : (m x k), where k is the rank of A
span<Q> = span<A> the columns of Q span the same space as the
columns of A, and the number of columns of Q is the rank of A.
Q satisfies: Q'*B*Q = I,