Add the verdi data upf content command

[sphuber]

Fixes #2467

This will print the bare contents of the file behind CifData nodes

[coveralls]

Coverage decreased (-0.03%) to 69.678% when pulling a3b7f3d on sphuber:fix_2467_verdi_data_cif_content into 69dbdb2 on aiidateam:provenance_redesign.

[giovannipizzi]

Ok, even if at some point we should start deprecating the methods returning a path and provide instead (in this case in the SinglefileData class) an open method