In step 3 the second correction to the momentum is determined. For gases the
RHS has already been determined in mafillsmv1rhs.f, for liquids a term containing
has to be added to the value calculated in mafillv1rhs.f (this is done in
mafillv2rhs.f and e_c3d_v2rhs.f). The lumped
equations are solved and the solution is added to
(again in fields v(1..nk,1), v(1..nk,2) and v(1..nk,3)).