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

Add a fwd include file, and a explicit template
instantiation definition in own compilation unit.

This decouples ogs.cpp from GWF process.