eigs behaviour for small matrices

[Jordi Gutiérrez Hermoso]

While porting Matlab code, I noticed that someone had used eigs for a
2x2 matrix, which produces an error to use eig instead. I patched the
code to do that, but I inadverently introduced a bug because I forgot
that eigs sorts its eigenvalues but eig does not.

Should eigs be patched to call eig for small matrices and just sort
them? Also, for the case of when you need all eigenvalues, should eigs
just silently call eig instead of warning about it?

- Jordi G. H.