Clarified cosmology dependence in morris av. Closes #568

This fixes #568 (closed).

New version behaves exactly the same as previous one, but with more sensible cosmology factors.

src/hydro/PressureEnergyMorrisMonaghanAV/hydro.h

@@ -640,14 +640,14 @@ __attribute__((always_inline)) INLINE static void hydro_prepare_force(
 
   /* Artificial viscosity updates */
 
-  /* TODO: Actually work out why this cosmology factor is correct
-   * and update the SPH / cosmology theory documents. */
+  /* We perform all of this in physical space. */
+  const float h_inv_physical = cosmo->a_inv * h_inv;
+  const float soundspeed_physical = cosmo->a_factor_sound_speed * soundspeed;
 
-  /* We divide by a^2 here to make this transform under cosmology the
-   * same as the velocity (which in SWIFT has an extra 1/a^2 factor.
-   * See the cosmology theory documents for more information. */
+  /* Decay rate */
   const float inverse_tau =
-      (hydro_props->viscosity.length * cosmo->a2_inv) * soundspeed * h_inv;
+      hydro_props->viscosity.length * soundspeed_physical * h_inv_physical;
+  /* Source term (div v) is already in physical co-ordinates for this scheme */
   const float source_term = -1.f * min(p->density.div_v, 0.f);
 
   /* Compute da/dt */