Doscar class doesn’t handle simple DOSCAR outputs well

[bjmorgan]

The DOSCAR class currently raises an error for DOSCAR files that do not include pDOS data.

e.g. https://github.com/bjmorgan/vasppy/issues/13

It would be helpful if the parser checks whether there is pDOS data present and can report appropriately. And / or allows simple DOS datasets to be loaded.