Add ability to specify license information

[GraemeWatt]

The JSON schema defining a HEPData submission has provision for a data_license (comprising a name, url and optional description) for individual tables. It also supports a license for additional resources. An example of use is given in submission.yaml used in TestHEPSubmission.zip and in the submission documentation. By default, the CC0 license applies, as explained in the HEPData Terms of Use.

The add_additional_resource function of hepdata_lib should be extended to accept an optional license argument (a Python dictionary containing name, url and optional description). The Table class should also be extended to include a new method add_data_license taking arguments name, url and optional description.

See also HEPData/hepdata#740.

[GraemeWatt]

Documentation should be added to hepdata_lib explaining how to add a license to either an additional resource file or a data table. As an example, the CC BY 4.0 license could be used.

name: CC BY 4.0
url: https://creativecommons.org/licenses/by/4.0/
description: This license enables reusers to distribute, remix, adapt, and build upon the material in any medium or format, so long as attribution is given to the creator.

[clelange]

Addressed in #257