Further cleaning of Pull kernel code

Implementing some suggestions by GenAI

Author: erikvansebille

docs/user_guide/examples_v3/tutorial_interaction.ipynb

@@ -77,20 +77,27 @@
     "    interaction_distance = 0.5\n",
     "    velocity = -0.04  # predefined attracting velocity\n",
     "\n",
-    "    lon_a = particles.lon[np.where(particles.attractor)[0]]\n",
+    "    # Boolean mask and coordinates of attractors\n",
+    "    attractor_mask = particles.attractor.astype(bool)\n",
+    "    lon_a = particles.lon[attractor_mask]\n",
+    "    lat_a = particles.lat[attractor_mask]\n",
+    "\n",
+    "    # Pairwise differences and distances (n_attractors × n_particles)\n",
     "    dx = particles.lon - lon_a[:, None]\n",
-    "    lat_a = particles.lat[np.where(particles.attractor)[0]]\n",
     "    dy = particles.lat - lat_a[:, None]\n",
+    "    distances = np.sqrt(dx**2 + dy**2)\n",
     "\n",
-    "    distances = np.sqrt(dx**2 + dy**2)  # TODO use np.linalg.norm?\n",
+    "    # Mask dx, dy by interaction range\n",
+    "    within = distances < interaction_distance\n",
+    "    dx = np.where(within, dx, 0.0)\n",
+    "    dy = np.where(within, dy, 0.0)\n",
     "\n",
-    "    dx = np.where(distances < interaction_distance, dx, 0.0)\n",
-    "    dy = np.where(distances < interaction_distance, dy, 0.0)\n",
+    "    with np.errstate(divide=\"ignore\", invalid=\"ignore\"):\n",
+    "        inv_dist = np.where(distances > 0, 1.0 / distances, 0.0)\n",
+    "        dx_norm = dx * inv_dist\n",
+    "        dy_norm = dy * inv_dist\n",
     "\n",
-    "    dx_norm = np.where(distances > 0, dx / distances, 0)\n",
     "    particles.dlon += np.sum(dx_norm, axis=0) * velocity * particles.dt\n",
-    "\n",
-    "    dy_norm = np.where(distances > 0, dy / distances, 0)\n",
     "    particles.dlat += np.sum(dy_norm, axis=0) * velocity * particles.dt"
    ]
   },