Update the signal velocity in more locations

This updates the signal velocity in more locations. It also:

• Removes the du_dt condition on the time-step in the pressure based schemes (we have never seen this make a difference and ruins the performance of runs with cooling)
• Updates the drift of the pressure-energy schemes to include a factor of u_new / u_old as this updates the pressures to be more consistent with the new value of the internal energy.

@lhausammann can you please make sure you are happy with the changes to the pressure schemes.

I think it makes sense to include a change in the pressures. For the condition on du/dt, I have no opinion. In the worst case, I can still include it in the grackle cooling.

Thanks for taking care of it. Let's see what it looks like on our favourite case...

merged

mentioned in commit 0cc08e36