from the two classes of coupled term.

[Coupling] Changed the members of the coupled term,

 which only contains the solutions of the coupled equations.

pointer to StaggeredCouplingTerm

 for the staggered scheme.

coupled terms.

[PCS] set the shape function order from config assuming all the primary variables use the same order

* context: class -> struct
* global vector from named fct: result_cache -> result
* cached 2ndary var: fixed member name