As there are no functors for computing the minimum and maximum,
we convert the std::max and std::min function pointers to
functors (which is not really nice.) Previously we were somehow
tricked into using std::greater and std::less, which of course do
return true or false and not what we need. Additionally, do more
excessive testing with different ranges.
We need to compute quite a few global reductions in the
Newton method of opm-autodiff. This commit adds the functionality
to compute several reductions combined using only one global
communication. Compiles and test succeeds with one or more process.
