ENH: Read eigenvectors and eigenvalues in the nwchem calculator

Migrated the relevant methods into the NWChem class as promised here https://listserv.fysik.dtu.dk/pipermail/ase-users/2017-June/003803.html. Also added the print keyword to the table of parameters in the documentation. The data read can be used also in the get_occupation_numbers() and get_eigenvalues() methods, which are currently not working for non-periodic calculations (not yet implemented).