Modify nbhood plugins to handle multi-time cubes

[bayliffe]

As an IMPROVER user I would like to be able to use the neighbourhood plugins for a cube that includes a time coordinate. At present this is not possible due to assumptions made about the data structure that allow for only a single time.

There is a discussion of this issue here: #1893

Acceptance criteria:

• all the neighbourhooding plugins are capable of natively handling cubes with a multi-valued time coordinate.
• unit tests and acceptance tests have been added to demonstrate this functionality.
• the interfaces should not change to avoid the need for modifications to user's current implementations.