Re-implementation of the accretion (gas only).

The accreated particles are injected when their accretion time is
lower than the current time. The accretion time is stored in the initial
condition file in place of the mass, with a negative value.

In cooling.c, we have multiply the min temperature by a factor of 2,
in order to avoid numerical problems when the cooling is very high.
This is not a real solution, and this should be better fixed.
The problem is due to the fact that because we use the entropy, if we
ensure that the temperature is higher than the min temperature for the
predicted time steps, this may not be respected, because the density also
varies.