#248 Add initial support for 'optional' queries (single statement only)

[bplatz]

This adds back support for optional statements in queries.

See new optional query test namespace for examples.

Note that multi-statement optional queries are not yet supported (throws a 'not implemented yet' exception). See issue #251

In the prior version of Fluree, optional statements were always wrapped inside a vector, even if just one statement. In this version single optional statements can just be declared (but can still be wrapped inside of a vector if desired). So the following two statements are equivalent:

{:select ['?name '?favColor]
 :where  [['?s :rdf/type :ex/User]
          ['?s :schema/name '?name]
          {:optional ['?s :ex/favColor '?favColor]}]}

{:select ['?name '?favColor]
 :where  [['?s :rdf/type :ex/User]
          ['?s :schema/name '?name]
          {:optional [['?s :ex/favColor '?favColor]]}]}

[zonotope]

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