pointer to StaggeredCouplingTerm

passing the coupling term via function argument.

this is already done at start of iteration

This has some follow-up changes.

Don't repeat typename in emplace_back.

 coupled processes.

coupled terms.

Also fix the order in the initializer list.

Remove default value.

... for the given variable and component ids instead of all element nodes,
which might not be a part of the associated mesh subset.