Catch up to opm-material reorganisation.

Specifically, file <opm/material/constants.hh> was renamed to
<opm/material/Constants.hpp> and this renaming must be reflected in
the CMake probes for opm-material.
Merge pull request #391 from bska/find-opm-material

[Build System] Catch up to opm-material reorganisation.
fix the averages wrt negative values and divisions by zero

it could be discussed whether it's better to throw a
std::runtime_error instead...
Merge pull request #390 from andlaus/fix_averages

fix the averages wrt negative values and divisions by zero