add COSMICTIME option (cosmictime.c) this allows to compute the cosmic time based on the scaling factor. This as been introduced to be able to compute the age of a star based on its associated scaling factor.

chimie.c:

• u_lt is now properly defined, base on the system of unit. u_lt = -log10(All.UnitTime_in_Megayears*1e6);
• add the correct way of deciding if a stellar particle has stars in a mass range where metals can be ejected. -> do_it

cooling.c

• now ok for comobile integration (in fact, it was already ok)

git-svn-id: https://svn.epfl.ch/svn/gasp@18 67b3a02f-582b-4e65-a7f6-2b87bd0cff7a