Improve filter for queries that return single results

[ngonzalezpazFC]

I drop this idea maybe for a future feature implementation regarding QuerySpec.
The method filter(QuerySpec qs) returns a List, even if searching for a single result. It would be useful for users if there is a method(filterSingleResult or sth) that returns an Optional instead of a List, to avoid handling a list. Also, as an additional feature it should throw an exception if more than one result was found.