[PL] Move process_variables parsing in own process

ProcessLib/GroundwaterFlow/CreateGroundwaterFlowProcess.cpp

ProcessLib/HT/CreateHTProcess.cpp

ProcessLib/HeatConduction/CreateHeatConductionProcess.cpp

ProcessLib/HydroMechanics/CreateHydroMechanicsProcess.cpp

ProcessLib/LiquidFlow/CreateLiquidFlowProcess.cpp

ProcessLib/RichardsFlow/CreateRichardsFlowProcess.cpp

...essLib/SmallDeformation/CreateSmallDeformationProcess.cpp

ProcessLib/TES/CreateTESProcess.cpp

...ib/TwoPhaseFlowWithPP/CreateTwoPhaseFlowWithPPProcess.cpp

ProcessLib/Utils/ProcessUtils.cpp