bug fix to SA function (#39)

Author: jtgasparik

src/aero_reps/aero_rep_single_particle.c

@@ -223,6 +223,9 @@ void aero_rep_single_particle_get_interface_surface_area__m2(
   int phase_model_data_id_first = -1;
   int phase_model_data_id_second = -1;
   double radius;
+  int i_part = aero_phase_idx_first / TOTAL_NUM_PHASES_;
+  aero_phase_idx_first -= i_part * TOTAL_NUM_PHASES_;
+  aero_phase_idx_second -= i_part * TOTAL_NUM_PHASES_;
 
   // Find the layer each phase (first and second) exist in 
   int i_phase_count = 0;

test/unit_aero_rep_data/test_aero_rep_single_particle.c

@@ -180,8 +180,8 @@ int test_surface_area_layer(ModelData * model_data, N_Vector state) {
   for( int i = 0; i < N_JAC_ELEM+2; ++i ) partial_deriv[i] = 999.9;
 
   aero_rep_get_interface_surface_area__m2(model_data, AERO_REP_IDX,
-                                AERO_PHASE_IDX_1 - NUM_AERO_PHASE, 
-                                AERO_PHASE_IDX_2 - NUM_AERO_PHASE, 
+                                AERO_PHASE_IDX_1, 
+                                AERO_PHASE_IDX_2, 
                                 &eff_sa, &(partial_deriv[1]));
 
   // calculate the volume density of the layer interface