A method 'density_matrix()' for evaluating a density matrix of a given shell
has been added to class ProjectorShell. It requires an ElectronicStructure
object as an input an by default produces a site- and spin-diagonal
part of the density matrix using the Fermi-weights obtained directly from VASP.
Ideally, this density matrix should coincide with the one calculated
within VASP itself (inside the LDA+U module).
Corresponding sanity test has been added, which shows only that the
calculation does not crash. Real numerical tests are needed.
When a ProjectorShell is created it creates a view of the full
projector array with orbital 'ilm' and band 'ib' indices interchanged.
The reason for this is that this corresponds more naturally to the
definition of the projector P_{m\nu} and also allows for multiplications
of projector matrices without additional transposition.
The tests have been modified accordingly.
