Work on heat conduction and thermal conductivity (FS #172)

- added somerton.hh in fluidmatrixinteractions/2p which provides a model
  for the effective thermal conductivity (static class)
- removed *matrixHeatFlux() methods in waterairspatialparams, introduced
  thermalConductivitySolid() instead
- added property ThermalConductivityModel, which is set as default to
  Somerton
- added effThermalConductivity and calculateEffThermalConductivity
  methods, thermal conductivities are averaged harmonically

Reviewed by Melanie


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10049 2fb0f335-1f38-0410-981e-8018bf24f1b0