Account for gravity in well connection flux calculation

We previously ignored effects of gravity in the calculation of the well
connection fluxes (i.e., perforation fluxes).  This commit includes
those effects where appropriate.

opm/core/pressure/tpfa/cfs_tpfa_residual.c

@@ -1090,7 +1090,7 @@ compute_wflux(int                        np    ,
 {
     int           w, c, i, p;
     double        pw, dp, t;
-    const double *pmob;
+    const double *pmob, *wdp;
 
     struct Wells          *W;
     struct CompletionData *cdata;
@@ -1101,13 +1101,14 @@ compute_wflux(int                        np    ,
 
     W     = wells->W;
     cdata = wells->data;
+    wdp   = cdata->wdp;
 
     for (w = i = 0; w < W->number_of_wells; w++) {
         pw = wpress[w];
 
         for (; i < W->well_connpos[w + 1]; i++) {
             c  = W->well_cells[ i ];
-            dp = pw - cpress[c];
+            dp = pw + wdp[ i ] - cpress[c];
 
             if (dp > 0) { pmob = cdata->phasemob + (i * np); } /* w->c */
             else        { pmob = pmobc           + (c * np); } /* c->w */