Add main table for uspto plugin

[AggelosMargkas]

Initialize new plugin support of US Patent Grant Bibliographic data. Add tables global variable with the first tables of the data schema.

• Add contents of the main table "us_patent" for the uspto plugin.
• Add properties that that have 1 to 1 relationship, with respect to the DTD rules of the dataset.

[dspinellis]

Nice progress!

• [ ] Please squash the two commits into a single clean one.
• [ ] Identify yourself in the copyright message as the plugin's author.
• [ ] Ensure you've run black and pylint

Prioritize importing the main table over adding more.

[AggelosMargkas]

• [x] Squash the two commits into one.
• [x] Identify myself as the author.
• [x] Run black and pylint command as described in the documentation.

[dspinellis]

Well done!