Hydrostatic halo in equilibrium in an isothermal potential. Running
for 10 dynamical times.

To make the initial conditions we distribute gas particles randomly in
a cube with a side length twice that of the virial radius. The density
profile of the gas is proportional to r^(-2) where r is the distance
from the centre of the cube.

The parameter v_rot (in makeIC.py and hydrostatic.yml) sets the circular
velocity of the halo, and by extension, the viral radius, viral mass,
and the internal energy of the gas such that hydrostatic equilibrium
is achieved.

To run this example, make such that the code is compiled with either
the isothermal potential or softened isothermal potential set in
src/const.h. In the latter case, a (small) value of epsilon needs to
be set in hydrostatic.yml.  ~1 kpc should work well.

The plotting scripts produce a plot of the density, internal energy
and radial velocity profile for each snapshot and divides the profile
by the expected profile. 

The script test_energy_conservation.py shows the evolution of energy
with time. These can be used to check if the example has run properly.

