Added tune for X coordinate in tracker [by request of ZW,WW]
the defaults for protons can be:

    TRMCFFKEY.MCtuneDs[1] =  2e-4;
    TRMCFFKEY.MCtuneDs[0]  = -14e-4;

for He

    TRMCFFKEY.MCtuneDs[1] =  -3e-4;
    TRMCFFKEY.MCtuneDs[0]  = 0;    //not yet defined


Note no backward compatibility (intentional)