[PL] Common functions. Rename x_dot to x_prev

ProcessLib/AbstractJacobianAssembler.h

ProcessLib/AnalyticalJacobianAssembler.cpp

ProcessLib/AnalyticalJacobianAssembler.h

ProcessLib/Assembly/ParallelVectorMatrixAssembler.cpp

ProcessLib/Assembly/ParallelVectorMatrixAssembler.h

ProcessLib/AssemblyMixin.h

ProcessLib/CentralDifferencesJacobianAssembler.h

ProcessLib/CompareJacobiansJacobianAssembler.h

ProcessLib/ForwardDifferencesJacobianAssembler.h

ProcessLib/LocalAssemblerInterface.cpp

ProcessLib/LocalAssemblerInterface.h

ProcessLib/Process.cpp

ProcessLib/Process.h

ProcessLib/VectorMatrixAssembler.cpp

ProcessLib/VectorMatrixAssembler.h