Correctly initialise the signal velocity of the SPH particles in the Minimal...

Correctly initialise the signal velocity of the SPH particles in the Minimal and Pressure-Entropy cases.

src/hydro/Minimal/hydro.h

@@ -279,7 +279,7 @@ __attribute__((always_inline)) INLINE static void hydro_reset_acceleration(
   /* Reset the time derivatives. */
   p->u_dt = 0.0f;
   p->force.h_dt = 0.0f;
-  p->force.v_sig = p->force.soundspeed;
+  p->force.v_sig = 0.0f;
 }
 
 /**

src/hydro/PressureEntropy/hydro.h

@@ -308,7 +308,7 @@ __attribute__((always_inline)) INLINE static void hydro_reset_acceleration(
   p->force.h_dt = 0.0f;
 
   /* Reset maximal signal velocity */
-  p->force.v_sig = p->force.soundspeed;
+  p->force.v_sig = 0.0f;
 }
 
 /**