Renamed heatConductivity() methods to thermalConductivity() according to the...

Renamed heatConductivity() methods to thermalConductivity() according to the name commonly employed in the fluid systems.
Added return function for thermalConductivity in the 2p2cnivolumevariables - this is required for some coupled models and does not add any new objects (addresses static function of fluidsystem)

Reviewed by Melanie

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9568 2fb0f335-1f38-0410-981e-8018bf24f1b0