[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.

renamed struct for type of element from FEMElemType to CellType and renamed virtual method getFEMType() to getCellType() in Elemnts class hierarchy