[porousmflow] Introduce compositional and immisible folders
* The immiscible folder contains files common to immiscible porousmediumflow. It now contains a general local residual for np flow * The compositional folder contains files common to the compositional porousmediumflow models. It now contains the privarswitch base class.
dumux/porousmediumflow/implicit/primaryvariableswitch.hh → dumux/porousmediumflow/compositional/primaryvariableswitch.hh
dumux/porousmediumflow/1p/implicit/localresidual.hh → dumux/porousmediumflow/immiscible/localresidual.hh