Add the plotting drho function to Davidson algorithm and a example H2O-PLOTRHO.


git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@11233 c92efa57-630b-4861-b058-cf58834340f0
small fix to the previous update


git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@11234 c92efa57-630b-4861-b058-cf58834340f0