Precision sections being generated for workflow entries

@ndaelman @lucamghi right now, the section Precision in results is being generated automatically for all calculations. I'd say this behavior only applies for SinglePoint calculations.

In fact, the way of resolving the k_line_density and basis_set sections and quantities inside Precision is to scan for entry_archive.run.method or run.calculation, which do not necessarily have to exist in workflow entries.

We should then move the generation of Precision into a very simple check: does run.method and run.calculation exist? If so, generate and populate Precision.

I can handle this if you quickly agree with the patch.