Add functionality in Procar class

Add convenient properties and method to the Procar class in order to get the number of k-points, the number of bands and to get the projection of bands on elements.
The method `get_projection_on_elements(s)` was written from the same method in the BandStructure class.
The PROCAR file is parsed faster than the vasprun.xml file with the `parse_projected_eigen` option.
Germain