Fixes and first version of test suite

• Add some documentation.
• Implement rough metadata conversion tests using pytest (to be refined and improved).
• Create CI job running pytest.
• Support 'funder' term in CodeMeta file
• Some fixes of bugs encountered while creating tests. In particular:
  • The CodeMeta generator creates a standard metadata file containing some values as single elements and not lists, which was not handled by FACILE-RS. For example, this (obtained from CodeMeata generator) was not accepted:

"contributor": {
        "@type": "Person",
        "familyName": "Contributor",
        "givenName": "One"
}

While this was:

"contributor": [
    {
        "@type": "Person",
        "familyName": "Contributor",
        "givenName": "One"
    }
]

• Better handling of the CodeMeta standard. For example, while 'funding' is of type 'Grant' in the schema.org standard, it is of type 'Text' in the Codemeta standard, and FACILE-RS only supported the 'Grant' type. This has been fixed.