Moved some more hydro parameters as optional

Some more parameters flagged as optional.

examples/CosmoVolume/cosmoVolume.yml

@@ -37,10 +37,8 @@ Statistics:
 SPH:
   resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
   delta_neighbours:      0.1      # The tolerance for the targetted number of neighbours.
-  max_ghost_iterations:  30       # Maximal number of iterations allowed to converge towards the smoothing length.
   max_smoothing_length:  0.705    # Maximal smoothing length allowed (in internal units).
   CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
-  max_volume_change:     2.       # Maximal allowed change of volume over one time-step
 
 # Parameters related to the initial conditions
 InitialConditions: