Fix transition pair absence

[gavinevans]

Addresses https://github.com/metoppv/mo-blue-team/issues/1293

Description
This PR addressed a minor issue that can occur in spin-up where the inputs for a specified transition in the SpatialMorphing plugin are both absent. For example, if the transitions dictionary is defined as below, but only source_c is available, the SpatialMorphing plugin will currently fail, as a transition is defined, but neither of the source pair is available. This PR adds more leniency, so that if source_a and source_b are both absent then the plugin will fallback to source_c.

{
    "transitions": [
        {
            "source_a": "source_a",
            "source_b": "source_b",
            "start_forecast_period_minutes": 60,
            "end_forecast_period_minutes": 240
        }
}

Testing:

• Ran tests and they passed OK
• Added new tests for the new feature(s)

[mo-jbeaver]

I've added some comments below, but all tests ran successfully.
Additionally, is there a fall back for it source_b isn't available and we can't default back to source_a as it is not available anymore (i.e. If it doesn't produce forecasts at that lead time.)?

[maxwhitemet]

Happy with the changes made. Approved 👍

[mo-jbeaver]

Added a few more questions below for some clarification, but all the tests passed successfully.

[mo-jbeaver]

Happy with all the changes, thanks.