VACF for gruop

Follow the instructions in section 6.15.

http://lammps.sandia.gov/doc/Section_howto.html#howto_15

this was also at the bottom of the vacf compute documentation page.

"Output info:

This compute calculates a global vector of length 4, which can be accessed by indices 1-4 by any command that uses global vector values from a compute as input. See this section for an overview of LAMMPS output options"