rename totalConcentration to phaseConcentration, because the former
can also be defined as the sum of all concentrations inside phases of
a component

remove the calculateMass() methods from the 2p and the 2p2c models and
replace them by a generic globalStorage() method in the box model base
class. the 2p2c model also gets a globalPhaseStorage(phaseIndex) method

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@4005 2fb0f335-1f38-0410-981e-8018bf24f1b0