Evolve gas at the static equilibrium in an NFW potential for several dynamical times.
This test allows to compare the impact of different hydro solvers on the initial 
hydrostatic equilibrium.

To run SWIFT configured with the sphenix hydro solver:
  ./configure --with-ext-potential=nfw --with-hydro=sphenix  
  
To run SWIFT configured with the gadget2 hydro solver:
  ./configure --with-ext-potential=nfw --with-hydro=gadget2 --disable-hand-vec   

To run SWIFT configured with the gizmo (MFV) hydro solver:
  ./configure --with-ext-potential=nfw --with-hydro=gizmo-mfv --with-riemann-solver=exact