Minor refactoring changes to GIZMO-MFM to allow use with the new hydro function configuration.

src/hydro/GizmoMFM/hydro.h

@@ -421,8 +421,7 @@ __attribute__((always_inline)) INLINE static void hydro_part_has_no_neighbours(
 /**
  * @brief Prepare a particle for the gradient calculation.
  *
- * The name of this method is confusing, as this method is really called after
- * the density loop and before the gradient loop.
+ * This function is called after the density loop and before the gradient loop.
  *
  * We use it to set the physical timestep for the particle and to copy the
  * actual velocities, which we need to boost our interfaces during the flux
@@ -433,7 +432,7 @@ __attribute__((always_inline)) INLINE static void hydro_part_has_no_neighbours(
  * @param xp The extended particle data to act upon.
  * @param cosmo The cosmological model.
  */
-__attribute__((always_inline)) INLINE static void hydro_prepare_force(
+__attribute__((always_inline)) INLINE static void hydro_prepare_gradient(
     struct part* restrict p, struct xpart* restrict xp,
     const struct cosmology* cosmo) {
 
@@ -446,6 +445,18 @@ __attribute__((always_inline)) INLINE static void hydro_prepare_force(
   hydro_velocities_prepare_force(p, xp);
 }
 
+/**
+ * @brief Resets the variables that are required for a gradient calculation.
+ * 
+ * This function is called after hydro_prepare_gradient.
+ * 
+ * @param p The particle to act upon.
+ * @param xp The extended particle data to act upon.
+ * @param cosmo The cosmological model.
+ */
+__attribute__((always_inline)) INLINE static void hydro_reset_gradient(
+    struct part *restrict p) {}
+
 /**
  * @brief Finishes the gradient calculation.
  *
@@ -461,6 +472,28 @@ __attribute__((always_inline)) INLINE static void hydro_end_gradient(
 
   hydro_gradients_finalize(p);
 
+#ifdef GIZMO_LLOYD_ITERATION
+  /* reset the gradients to zero, as we don't want them */
+  hydro_gradients_init(p);
+#endif
+}
+
+/**
+ * @brief Prepare a particle for the force calculation.
+ *
+ * This function is called in the extra_ghost task to convert some quantities coming
+ * from the gradient loop over neighbours into quantities ready to be used in the
+ * force loop over neighbours.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ * @param cosmo The current cosmological model.
+ */
+__attribute__((always_inline)) INLINE static void hydro_prepare_force(
+    struct part* restrict p, struct xpart* restrict xp,
+    const struct cosmology* cosmo) {
+
+  /* Initialise values that are used in the force loop */
   p->gravity.mflux[0] = 0.0f;
   p->gravity.mflux[1] = 0.0f;
   p->gravity.mflux[2] = 0.0f;
@@ -470,11 +503,6 @@ __attribute__((always_inline)) INLINE static void hydro_end_gradient(
   p->conserved.flux.momentum[1] = 0.0f;
   p->conserved.flux.momentum[2] = 0.0f;
   p->conserved.flux.energy = 0.0f;
-
-#ifdef GIZMO_LLOYD_ITERATION
-  /* reset the gradients to zero, as we don't want them */
-  hydro_gradients_init(p);
-#endif
 }
 
 /**