Slimming down of foreign gpart + reduced comm size

Here are the changes to improve the foreign memory usage of gparts and help with the related communications.

This includes:

• Creation of two new particle types. One for the regular foreign gpart and one for the gpart when running FOF.
• Modification of the gravity cache construction to use the foreign particle type when acting on a foreign cell.
• Usage of the new packing task to fish out the fields we want. (No need to unpack anything however)
• Modification of the gpart and fof communications.
• Modification of the construction of the foreign buffers to accommodate both types.

All the physics changes are the same as what was tried in !1318 (closed). The difference is that the packing is done here by hand and that the case of FOF is correctly handled.

src/gravity/Default/gravity.h

@@ -29,6 +29,27 @@
 #include "kernel_gravity.h"
 #include "minmax.h"
 
+/**
+ * @brief Copy the content of a #gpart into a #gpart_foreign.
+ */
+__attribute__((always_inline)) INLINE static void gravity_foreign_copy(
+    struct gpart_foreign* packed, const struct gpart* full) {
+
+  packed->x[0] = full->x[0];
+  packed->x[1] = full->x[1];
+  packed->x[2] = full->x[2];
+
+  packed->mass = full->mass;
+
+  packed->time_bin = full->time_bin;
+
+  packed->type = full->type;
+
+#ifdef SWIFT_DEBUG_CHECKS
+  packed->ti_drift = full->ti_drift;
+#endif
+}
+
 /**
  * @brief Returns the mass of a particle
  *
@@ -55,6 +76,22 @@ __attribute__((always_inline)) INLINE static float gravity_get_softening(
   return grav_props->epsilon_DM_cur;
 }
 
+/**
+ * @brief Returns the current co-moving softening of a foreign particle
+ *
+ * Note that in this basic gravity scheme, all particles have
+ * the same softening length.
+ *
+ * @param gp The particle of interest
+ * @param grav_props The global gravity properties.
+ */
+__attribute__((always_inline)) INLINE static float
+gravity_get_softening_foreign(const struct gpart_foreign* gp,
+                              const struct gravity_props* restrict grav_props) {
+
+  return grav_props->epsilon_DM_cur;
+}
+
 /**
  * @brief Add a contribution to this particle's potential from the tree.
  *