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 cooling.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.

While the system is initially in hydrostatic equilibrium, the cooling
of the gas means that the halo will collapse.

To run this example, make such that the code is compiled with either
the isothermal potential or softened isothermal potential, and
'const_lambda' cooling, set in src/const.h. In the latter case, a
(small) value of epsilon needs to be set in cooling.yml.  0.1 kpc
should work well.

The plotting scripts produce a plot of the density, internal energy
and radial velocity profile for each
snapshot. test_energy_conservation.py shows the evolution of energy
with time. These can be used to check if the example has run properly.