Adding info on Interaction Kernel in migration guide

Author: erikvansebille

docs/user_guide/examples/tutorial_interaction.ipynb

@@ -55,7 +55,7 @@
    "outputs": [],
    "source": [
     "def Pull(particles, fieldset):\n",
-    "    \"\"\"Kernel that \"pulls\" all neighbor particles\n",
+    "    \"\"\"Kernel that \"pulls\" all neighbour particles\n",
     "    toward the attracting particle with a constant velocity\"\"\"\n",
     "    interaction_distance = 0.5\n",
     "    velocity = -0.04  # predefined attracting velocity\n",

docs/user_guide/v4-migration.md

@@ -14,6 +14,7 @@ Version 4 of Parcels is unreleased at the moment. The information in this migrat
 - The `particle` argument in the Kernel signature has been renamed to `particles`.
 - `math` functions should be replaced with array compatible equivalents (e.g., `math.sin` -> `np.sin`). Instead of `ParcelsRandom` you should use numpy's random functions.
 - `particle.depth` has been changed to `particles.z` to be consistent with the [CF conventions for trajectory data](https://cfconventions.org/cf-conventions/cf-conventions.html#trajectory-data), and to make Parcels also generalizable to atmospheric contexts.
+- The `InteractionKernel` class has been removed. Since normal Kernels now have access to _all_ particles, particle-particle interaction can be performed within normal Kernels.
 
 ## FieldSet