improve stability of eigenValues2dImpl() by reformulating radicand
requested to merge joscha.podlesny/dune-common:feature/eigenValues2dImpl_stability_patch into master
matrix A = (a00 a01; a10 a11) characteristic polynomial: p(x) = x^2 - (a00 + a11)x + (a00a11 - a01*a10)
current implementation: eigenvalues (roots) by pq-formula: x_{1/2} = (a00 + a11)/2 +/- \sqrt{ ((a00 + a11)/2)^2 - (a00a11 - a01a10)} stability issue: symmetric matrix close to identity may lead to negative radicand due to round-off errors
fix: reformulate radicand such that subtraction occurs as early as possible, i.e. x_{1/2} = (a00 + a11)/2 +/- \sqrt{ ((a00 - a11)/2)^2 + a01*a10 }