Merge branch 'manyPreprocessing' into 'master'

Formulation fails when calling Formulation::pre() many times

See merge request !18

src/problem/Formulation.cpp

@@ -897,6 +897,7 @@ namespace gmshddm
       MPI_Barrier(PETSC_COMM_WORLD);
 
       // get the local interface unknowns for each processor
+      _rhs.clear();
       for(auto idom = 0ULL; idom < _surface.size(); ++idom) {
         if(mpi::isItMySubdomain(idom)) {
           for(auto it = _surface[idom].begin(); it != _surface[idom].end(); ++it) {