Extend gUFO with inverse object properties in a separate module (gufoi)

[tgoprince]

This PR makes the following changes:

• Adds a module that extends gUFO by providing inverse object properties
• Adds 3 boilerplate files users can leverage to learn and using gUFO
• Updates the official documentation to mention the previous two items
• Removes unused imports from the gufo.ttl file (for foaf, schema.org)

Accepting this PR requires that we create the following purls that should point to the ttl files added in this PR:

• http://purl.org/nemo/gufo/inverse
• http://purl.org/nemo/gufo/boilerplates/gufo-import
• http://purl.org/nemo/gufo/boilerplates/gufo-gufoi-import
• http://purl.org/nemo/gufo/boilerplates/car-rental

[tgoprince]

@jpalmeida

[tgoprince]

Please find below the OntoUML model I used for the car rental example.

I'm wondering whether we should add it to the repository as well.