FEATURE: Make the abinit BandStructure job follow the Vasp BandStructure job

[davidwaroquiers]

In Vasp, NonScfSetGenerator is defined by a reciprocal_density (for uniform band structures) or by a line_density (for line band structures). It would be nice to be able to use the same convention in Abinit. We should keep the possibility to use the "usual" abinit way also (uniform band structures defined by kppa, and line band structures defined by ndivsm) as abinit users may be more inclined to use that convention.