Implement quality test on ELF data

[jhidding]

The integral of the ELF data should give a (material dependent) constant.

[jhidding]

Annemarie: Thomas came up with this one; maybe you know what this is supposed to mean?

[atheulings]

The f-sum rule (Abril, 1998 eq. 8) should be true:

Where n is the atomic density of the target.
In the case of a compound, the f-sum rule will be equal to the average number of electrons per atom.