What first-order connections do regions A and B have in common?

See #21

The `Connections` class should have a method for answering the question:

- What first-order connections do regions A and B have in common?

In which the user should be able to specify whether the connections to be considered in common are with A and B as the input nodes targeting the same common node, output nodes targeted from the same common node, or either ("both").

The `direct_connections` method in #21 should be used, where possible.

Read the Connections class and issue #21's direct_connections method first. Define the requested query for shared first-order connections when A and B are input nodes, output nodes, or either, reusing direct_connections where possible. Done means the method supports all three modes and returns the expected common connections.

Written by the indexing model from the issue text.