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.