Debug fixes for neighbour finding

Debug fixes for problems with neighbour-finding, no neighbours, and maximum smoothing lengths that came up in planetary test simulations.

Awaiting thorough tests before merging.

src/chemistry/EAGLE/chemistry.h

@@ -79,6 +79,22 @@ __attribute__((always_inline)) INLINE static void chemistry_end_density(
     const struct cosmology* cosmo) {}
 
 /**
+ * @brief Sets all particle fields to sensible values when the #part has 0 ngbs.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ * @param cd #chemistry_global_data containing chemistry informations.
+ * @param cosmo The current cosmological model.
+ */
+__attribute__((always_inline)) INLINE static void
+chemistry_part_has_no_neighbours(struct part* restrict p,
+                                 struct xpart* restrict xp,
+                                 const struct chemistry_global_data* cd,
+                                 const struct cosmology* cosmo) {
+  error("Needs implementing!");
+}
+
+/**
  * @brief Sets the chemistry properties of the (x-)particles to a valid start
  * state.
  *