multidimensional optimization

[Brian Gough]

I've changed my mind on this now, gsl_vector and gsl_matrix are useful
here. The multidimensional algorithms involve the linalg solvers (LU,
QR) which use gsl_vector and gsl_matrix so it makes sense to use them.

Brian Gough writes:
 > I would avoid using gsl_vector myself. There is a constraint that all
 > the arrays are of the same length, so I would make the struct enforce
 > this explicitly through a single parameter n. Also I prefer not to
 > introduce cross-dependencies between the directories where they aren't
 > essential.