Convert forecast period units to fix no overlap, and remove additional attributes

[James Warner (jwarner8)]

As per title, FastNet not working as it reports forecast_period in seconds, and overlap literally evaluates forecast_period numbers. This fix ensures that they use a common time unit.

Also adds some additional attributes to pop that prevent cube merging.

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• Documentation has been updated to reflect change.
• New code has tests, and affected old tests have been updated.
• All tests and CI checks pass.
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[github-actions]

[James Warner (jwarner8)]

Closing as these changes are already on trunk from a separate PR.