Support for arity and tokenarity

[margaretha]

Krill should support arity and tokenarity, which are unary operators in Annis QL.

• Arity adds a restriction about the number of child nodes directly dominated by a node.

• Tokenarity adds a restriction about the number of tokens covered by a node.

An example of Koral query serialization of arity is shown below.

 "attr": {
      "@type": "koral:term",
      "arity": {
        "@type": "koral:boundary",
        "max": 4,
        "min": 4
      }
    }

Until arity and tokenarity is fully supported, Krill should set a warning in the JSON-LD response.