Add ATLAS_CHECK_THREAD_SAFETY, use override final

a mutable that needed be a couple of static --> const static

and also override and final

In a case where we do not need to keep around return R changed

R = R.inverse();
 double chiSquared = residual.transpose()*R*residual;

to

double chiSquared = residual.transpose()*R.inverse()*residual;

Mentioning @amorley