disk_io='medium', that is, do not write to RAM, seems a more reasonable default
for non-scf calculations: in typical cases, it will use less RAM but not 
perform more I/O than disk_io='low' (current default)


git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@11116 c92efa57-630b-4861-b058-cf58834340f0