Find an instance based on query

Property definitions

popcorn $ Repository :: find
	# Find an instance based on `query`
	fun find(query: QUERY): nullable E is abstract
lib/popcorn/pop_repos.nit:178,2--179,47

popcorn $ MongoRepository :: find
	redef fun find(query) do
		var res = collection.find(query)
		if res == null then return null
		return deserialize(res.to_json)
	end
lib/popcorn/pop_repos.nit:323,2--327,4