add a new parameter keyword : StarsAllocFactor This allows to reduce the amount of memory used by the StP structure. Its value gives an extimation of the fraction of gas particles that will convert into stars

modifications in : allvars.h,begrun.c,allocate.c,read_ic.c

git-svn-id: https://svn.epfl.ch/svn/gasp@35 67b3a02f-582b-4e65-a7f6-2b87bd0cff7a