1. Context for MPI communication of external potential is fixed to wf.spincontext. 
    2. An error in computing displacements for MPI_Scatterv is fixed.
    3. At the end of SCF iterations under +Vext, before Vext is reversed, Vxc will not be updated under RPA mode.  
    
Change: 
    Now the output of response (-vext) command would be 0.5 * 1/(amplitude of vext) * ( rho(+Vext) - rho(-Vext) ). In this way the result is more naturally processed by WEST.


git-svn-id: http://qboxcode.org/svn/qb/branches/vext@1891 cba15fb0-1239-40c8-b417-11db7ca47a34