Remove the fastQuadratic option from bisection
In the new tnnmg implementation, the calling method is expected to know whether a functional is quadratic or not. In case of a quadratic functional, the caller will not call bisection anyway, but it will rather call a dedicated solver for quadratic problems. This patch also introduces the preprocessor variable USE_OLD_TNNMG. When this variable is defined, the old tnnmg implementation is reenabled. This will simplify running old and new codes side by side for a while, which I find helpful for debugging.