WIP: Grid arrays
Major refactorings, step 1.
The objective of this is to simplify the state of the main GPAW objects so things like calc.set(), restarting, etc., won't be so buggy.
Objects are to be split into smaller, simpler objects. The main feature for now is a new object, GridArray, which is an array plus a grid descriptor. Also Density and Hamiltonian put their densities/potentials/energies into (still) small wrapper classes.
Very WIP.