actions: the generated return task model for actions is confusing

[doudou]

Follow-up of the discussion in #37

Currently, Roby generates a specific task model for all actions. It is necessary for action coordination objects (scripts and actions) as the system designer rarely touches the toplevel task. For handwritten actions, it is plain confusing.

So, I would move to have the default returns be Roby::Task for "manual" actions, and use an automatically generated task model only for action scripts/action state machines.

In addition, we should validate that the value returned by an action is not abstract UNLESS it is specified (i.e. add a returns_abstract statement in the action definition). The vast majority of cases will NOT want an abstract task as root.