Multidata plugins

[MarioVilas]

Idea: add the ability for plugins to receive multiple objects in one shot instead of forking for each object. That way the plugin would be invoked once per stage execution.

Related: #268

[MarioVilas]

This isn't as simple as it sounds.

Changing the Notifier is quite easy, it can be done in a few minutes (I just tried). The tricky part then is handling the ACKs.

Here are some solutions I thought of:

• The ACK ID could be a list of IDs. This may introduce subtle type errors, so I don't like it.
• The ACK ID could be a hash of the list of IDs. I'm not quite comfortable with this either - the hash wouldn't correspond to any Entity.
• The ACK ID could be a concatenation of IDs (as it happens with Relationship). This sounds better. :)
• Create a new Entity to represent a Dataset. This seems like the best solution, but it's more work. It also solves the problem of how to call the plugins - instead of creating a new method, we can just use the same method with this new Entity type. To indicate that we want multiple objects in a plugin we return the Dataset type in get_accepted_types, just like we do with Relationship types.