various improvements

[EvanBldy]

Problem

• There's no functions to manage concepts.
• There's some missing tests.
• We can't specify the "for_entity" when creating a task type (Allow task.new_task_type() to specify an entity. #306).

Solution

• Add some functions to manage concepts.
• Add some missing tests.
• Add new parameter "for_entity" for gazu.task.new_task_type().