Move and refactor remove_base to unresolve function in iri_resolver

[mielvds]

This PR

• moves remove_base(base, iri) to iri_resolver.py and renames it to unresolve(iri, base) (in correspondence with resolve())
• refactor function to use standard lib urlparse and urlunparse
• migrate references to remove_base((base, iri))
• add unittests for unresolve(iri, base) in tests/test_iri_parser.py