feat: implement DifferenceFilter to compute the difference between two variables

[nicrie]

Description

Add a difference filter that computes $X-Y$ between two matched variables and registers it in the filter registry.

What problem does this change solve?

New feature: enables simple variable differencing in transform pipelines.

What issue or task does this change relate to?

None.

Additional notes

Is the list of available filters auto-generated? Let me know where to update the source docs if necessary and I can add the difference filter there.

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[aaron-hopkinson]

Thanks for this. The filter looks good to me.

Would it also be possible to add a test, please? 🙂

[aaron-hopkinson]

Suggested change

	# (C) Copyright 2024 Anemoi contributors.
	# (C) Copyright 2026- Anemoi contributors.

[aaron-hopkinson]

I wonder if it's worth having an abs: bool = False argument as well (which optionally takes the absolute value of the difference)? I can imagine that there may cases where the absolute difference is useful (and prevents people having to think about which is X and which is Y)