[doc][doxygen] Cleanup for new materiallaw.

...erial/fluidmatrixinteractions/2p/datasplinemateriallaw.hh

dumux/material/fluidmatrixinteractions/2p/heatpipelaw.hh

...fluidmatrixinteractions/2p/interfacialarea/exponential.hh

...trixinteractions/2p/interfacialarea/polynomial2ndorder.hh

.../material/fluidmatrixinteractions/2p/splinemateriallaw.hh

dumux/material/fluidmatrixinteractions/2p/vangenuchten.hh

dumux/porousmediumflow/2p/boxmaterialinterfaces.hh

dumux/porousmediumflow/2p/saturationreconstruction.hh