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.