Added derivative of eigenvalues of symmetric rank2.  Fixes #3563

@jwpeterson - if i'm just wanting the eigenvalues, and not the vectors, wouldn't symmetricEigenvalues be more efficient?  I thought that was why LAPACK had the first argument "N" or "V".  Also, in dsymmetricEigenvalues i have to construct deigenvals, which inefficient if i don't actually need them.