Rework `Subscription` creation

[themightyoarfish]

Right now, the leaf subscriber classes accept a kinds parameter and use that to create a Subscription object to pass to the superclass initialiser.

It is weird that the base class initialiser's parameters are not a proper subset of the inheriting classes' initialisers.

Proposed Solution

1. accept kinds, tag, and subsample parameters in base Subscriber initialiser, and instantiate Subscription there.
2. … not sure what else

[themightyoarfish]

Improved Idea

Subscribers should be able to have multiple Subscriptions.

1. Subscriber has mapping of kinds to subscription objects
2. in handle_message() the appropriate Subscription is retrieved and the message delegated to it.
3. The Subscription then handles the message as before

[themightyoarfish]

Fixed in c76d11f

[themightyoarfish]

Actually not fixed, but subscriptions more versatile now.