todo

math library from gnu

* dlae2.f  has better handling of 2x2 eigenvalues than our qrstep.c

* Document Jacobi eigen function, in particular that it only works for
symmetric matrices.

* add support for nonsymmv left eigenvectors?:

  gsl_eigen_nonsymmv_params(const int lr)

  to specify whether we should compute left/right eigenvectors (or both).

  If they want both, we'll need a new function:

  gsl_eigen_nonsymmv_lr(..., evec_r, evec_l, ...)
  and
  gsl_eigen_nonsymmv_lr_Z(...)