Remove cell task counters ?

The cell structure contains counters for the number of density,force and gravity tasks. These are incremented atomically when we add a cell<->task link but they are never read or used anywhere in the code after that.

We also have a counter of all tasks which is the sum of all these three counters. This one is used (albeit in one single place where we only care whether its >0)

Do you see any reason to keep the individual task counters ? I was going to remove them.