[quad] reanimate support for quad precision arithmetic

Merged Bernd Flemisch requested to merge feature/reanimate-quad-support into master

Add CMake detection by copying FindQuadmath.cmake from opm-common/cmake/Modules at https://github.com/OPM/opm-common/, version 37da204.

Remove hack for GCC <= 4.6. Add Dune's IsNumber specialization.

Test by using quad as Scalar and recalculating the one-phase incompressible TPFA test with analytic Jacobian.

A quite annoying detail is that dumux/common/quad.hh has to be included before any other Dune module, at least I couldn't find a better way.

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