This example shows how to use cp.x together with pw.x 
to perform an electronic minimization at Gamma for SiO2.
Note that
- it works only for Gamma point, of course
- it is no longer needed to set "wf_collect" to .t.
- the number of bands should be the same for CP and PW
- "prefix" for pw.x = "prefix"_"ndr" or "prefix_ndw" for cp.x
