Correct the use of rshift to reduce the number of distance calculations.

Applies rshift (boundary conditions) correctly in DOPAIR1 + DOPAIR2 to reduce the number of spurious distance calculations.

src/runner_doiact.h

@@ -1037,9 +1037,9 @@ void DOPAIR1(struct runner *r, struct cell *ci, struct cell *cj, const int sid,
       struct part *restrict pj = &parts_j[sort_j[pjd].i];
       if (!part_is_active(pj, e)) continue;
       const float hj = pj->h;
-      const double dj = sort_j[pjd].d - hj * kernel_gamma - dx_max - rshift;
-      if (dj > di_max) continue;
-
+      const double dj = sort_j[pjd].d - hj * kernel_gamma - dx_max + rshift;
+      if (dj - rshift > di_max) continue;
+      
       double pjx[3];
       for (int k = 0; k < 3; k++) pjx[k] = pj->x[k] + shift[k];
       const float hjg2 = hj * hj * kernel_gamma2;
@@ -1449,9 +1449,9 @@ void DOPAIR2(struct runner *r, struct cell *ci, struct cell *cj) {
     /* Get a hold of the jth part in cj. */
     struct part *restrict pj = &parts_j[sort_j[pjd].i];
     const float hj = pj->h;
-    const double dj = sort_j[pjd].d - hj * kernel_gamma - dx_max - rshift;
-    if (dj > di_max) continue;
-
+    const double dj = sort_j[pjd].d - hj * kernel_gamma - dx_max + rshift;
+    if (dj - rshift > di_max) continue;
+    
     double pjx[3];
     for (int k = 0; k < 3; k++) pjx[k] = pj->x[k] + shift[k];
     const float hjg2 = hj * hj * kernel_gamma2;