Add functions for the heat capacity to the components air and co2, that will...

Add functions for the heat capacity to the components air and co2, that will be required by the decoupled NI-model. Enabled acess to these methods in the co2-brine-fluidsystem (only pure-phase heat capacities are used).
reviewed by Klaus

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12462 2fb0f335-1f38-0410-981e-8018bf24f1b0