What this MR does / why does DuMux need it:
Deprecates the function poroMechanicsCouplingContext() in the poroelastic coupling manager. The data that is needed in the poroelastic subdomain from the porous medium subdomain is all encapsulated in the vol vars. This is the object that one would extract from the context (which doesnt carry much more). Therefore, we introduce the function getPMFlowVolVars(element) which lets you obtain the corresponding volume variables without having to know what the underlying context looks like.
Also, it removes the problem pointer storage in the coupling manager, because these are already stored in the base class.