Added citations to headers

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@9880 f3b2605a-c512-4ea7-a41b-209d697bcdaa

potentials/ffield.reax.budzien

-Reactive MD-force field: PETN
+Reactive MD-force field: PETN: J. Budzien, A. P. Thompson, and S. V. Zybin, J. Phys. Chem., 113 13142 (2009).
  39       ! Number of general parameters                                        
    50.0000 !Overcoordination parameter                                          
     9.4514 !Overcoordination parameter                                          

potentials/ffield.reax.mattsson

-Reactive MD-force field
+Reactive MD-force field: general-purpose hydrocarbon parameterization: T. R. Mattsson, J. M. D. Lane, K. R. Cochrane, M. P. Desjarlais, A. P. Thompson, F. Pierce, and G. S. Grest, Phys. Rev. B, 81 054103 (2010).
  39       ! Number of general parameters                                        
    50.0000 !Overcoordination parameter                                          
     9.5469 !Overcoordination parameter