Parallel Vasp jobs should set NCORE automatically

[pmrv]

Vasp wiki recommends either 2 to the number of cores per socket or sqrt(cores). I guess we can just adopt either of these.

Ping @raynol-dsouza since he brought it up in the meeting today. :p

[Leimeroth]

Since NCORE can have very different optimal values depending on the architecture of the HPC system the implementation should take of not overwriting a manually set value.

[raynol-dsouza]

I'm still thinking about the right way of setting the NCORE flag. In addition to @Leimeroth's very valid point, one would also need to make sure that master jobs like the Murnaghan job don't break or underperform because of this.