get rid of matrixHeatFlux, rename heatCapacity

The 3pwateroil model still uses the method matrixHeatFlux from the spatial parameters. This should be made consistent with the other nonisothermal models, namely, by calculating the matrix heat flux in the flux variables using an effective thermal conductivity relation.

For the same reasoning, one should rename heatCapacity to solidHeatCapacity.

The test of the 2pnc model also still provides a function matrixHeatFlux in its spatial parameters. This should be removed.