ENH: Introduction of a new background_charge parameter in the GPAW calculator, new Jellium implementation

These changes introduce a new parameter in the gpaw calculator, background_charge, which is associated with a Jellium object when doing Jellium calculations.

This is a better way of correcting the problems with Jellium and dipole correction (better than !61 (closed)).

(If there is ever a need to implement some other type of background charge, the important thing is that those objects also have to have a .charge attribute, which is the total positive charge in that background.)