[misc] Fix some occurences of the old GET_PARAM macro yielding compiler erros

See merge request !620

This implies that all processes use the same interaction volume on the boundaries. We typically
use the o-method on boundaries, where this is guaranteed.

Feature/3p unify main files

See merge request !618

allows for primary variable gradients evaluation in an element for the box
scheme. The specialization for CC methods throws an exception.

Cleanup/next

See merge request !571