Adds v_sig calculation to force loop in ANARCHY-DU

This adds a max() in the force loop for the signal velocity, for ANARCHY-DU. This should hopefully help with some time integration problems we've been seeing, but should be considered a quick fix (note the comment) so that people can get on with XL related stuff.

MS: are you happy with this to go into the codebase? I don't like that it's masking an issue but I guess we're kinda up against it :)

src/hydro/AnarchyDU/hydro_iact.h

@@ -428,6 +428,12 @@ __attribute__((always_inline)) INLINE static void runner_iact_force(
   /* Get the time derivative for h. */
   pi->force.h_dt -= mj * dvdr * r_inv / rhoj * wi_dr;
   pj->force.h_dt -= mi * dvdr * r_inv / rhoi * wj_dr;
+
+  /* Update if we need to; this should be guaranteed by the gradient loop but
+   * due to some possible synchronisation problems this is here as a _quick
+   * fix_. Added: 14th August 2019. To be removed by 1st Jan 2020. (JB) */
+  pi->viscosity.v_sig = max(pi->viscosity.v_sig, v_sig);
+  pj->viscosity.v_sig = max(pj->viscosity.v_sig, v_sig);
 }
 
 /**
@@ -547,6 +553,11 @@ __attribute__((always_inline)) INLINE static void runner_iact_nonsym_force(
 
   /* Get the time derivative for h. */
   pi->force.h_dt -= mj * dvdr * r_inv / rhoj * wi_dr;
+
+  /* Update if we need to; this should be guaranteed by the gradient loop but
+   * due to some possible synchronisation problems this is here as a _quick
+   * fix_. Added: 14th August 2019. To be removed by 1st Jan 2020. (JB) */
+  pi->viscosity.v_sig = max(pi->viscosity.v_sig, v_sig);
 }
 
 /**