rename doxygen groups "fluidmatrixinteractions(laws|params)" to "FluidMatrixInteractions"

and improve the van Genuchten law
Refactor the twophase laws

They are now all generic capillary pressure laws, but with some
additional methods to make working with them easier. For this reason,
they have been moved up one directory to opm/material/fluidmatrixinteractions

Also, a unit test which ensures that all capillary pressure laws
conform to their respective APIs has been added.
Cosmetic change: remove the subdirectories for sources of the tests

that was a leftover of eWoms/Dumux...
add the three-phase capillary pressure law Eclipse uses by default
fluid-matrix interactions: API extension

- add methods to calculate the derivatives of the capillary pressures
  and relative permeabilities with regard to the phase saturations,
  temperature, absolute pressure and phase composition
- extend the unit test to enforce the above
- make the NullMaterial conform to the API and add it to the unit test
- introduce Opm::NullMaterialTraits for material laws that don't use
  any phase indices
rename "pcwn" to "pcnw" and "pcng" to "pcgn"

because it is p_n - p_w and p_g - p_n instead of the other way round.