Add iact for star formation

In GEAR, we need to compute the gas' velocity dispersion for the star formation law. Thus we need to add a few computation in the density loop.

In this merge request, I am only implementing empty functions. My student will work on it during this semester, therefore it will be a bit long to wait on his work.

The structure is the same than in the chemistry and everything is implemented for EAGLE and the default star formation.

src/star_formation/EAGLE/star_formation.h

@@ -626,4 +626,69 @@ INLINE static void starformation_print_backend(
           starform->max_gas_density_HpCM3);
 }
 
+/**
+ * @brief Finishes the density calculation.
+ *
+ * Nothing to do here. We do not need to compute any quantity in the hydro
+ * density loop for the EAGLE star formation model.
+ *
+ * @param p The particle to act upon
+ * @param cd The global star_formation information.
+ * @param cosmo The current cosmological model.
+ */
+__attribute__((always_inline)) INLINE static void star_formation_end_density(
+    struct part* restrict p, const struct star_formation* cd,
+    const struct cosmology* cosmo) {}
+
+/**
+ * @brief Sets all particle fields to sensible values when the #part has 0 ngbs.
+ *
+ * Nothing to do here. We do not need to compute any quantity in the hydro
+ * density loop for the EAGLE star formation model.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ * @param cd #star_formation containing star_formation informations.
+ * @param cosmo The current cosmological model.
+ */
+__attribute__((always_inline)) INLINE static void
+star_formation_part_has_no_neighbours(struct part* restrict p,
+                                      struct xpart* restrict xp,
+                                      const struct star_formation* cd,
+                                      const struct cosmology* cosmo) {}
+
+/**
+ * @brief Sets the star_formation properties of the (x-)particles to a valid
+ * start state.
+ *
+ * Nothing to do here.
+ *
+ * @param phys_const The physical constant in internal units.
+ * @param us The unit system.
+ * @param cosmo The current cosmological model.
+ * @param data The global star_formation information used for this run.
+ * @param p Pointer to the particle data.
+ * @param xp Pointer to the extended particle data.
+ */
+__attribute__((always_inline)) INLINE static void
+star_formation_first_init_part(const struct phys_const* restrict phys_const,
+                               const struct unit_system* restrict us,
+                               const struct cosmology* restrict cosmo,
+                               const struct star_formation* data,
+                               const struct part* restrict p,
+                               struct xpart* restrict xp) {}
+
+/**
+ * @brief Sets the star_formation properties of the (x-)particles to a valid
+ * start state.
+ *
+ * Nothing to do here. We do not need to compute any quantity in the hydro
+ * density loop for the EAGLE star formation model.
+ *
+ * @param p Pointer to the particle data.
+ * @param data The global star_formation information.
+ */
+__attribute__((always_inline)) INLINE static void star_formation_init_part(
+    struct part* restrict p, const struct star_formation* data) {}
+
 #endif /* SWIFT_EAGLE_STAR_FORMATION_H */