Add option for steps in Cube rolling windows

[mo-AndrewCreswick]

Description

This adds a step argument to iris.cube.Cube.rolling_window method to make use of the full functionality available in iris.util.rolling_window. A default argument of step=1 ensures backwards compatibility.

Unit tests added to test the aggregation over rolling windows with a step between windows and also to test that coordinates are handled correctly with a step. The behaviour of the window creation with steps is tested more thoroughly in the unit tests for iris.util.rolling_window.

I'm not sure if benchmarking is required for this PR so have not added the label for now.

Checklist

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• Included a What's New entry
• Incorporated type hints in any changed code
• Checked if tests need updating
• Checked if benchmarks need updating
• Checked if documentation needs updating
  • Docstrings (we love code examples!)
  • User Manual pages
• Checked if dependencies need updating
• Confirmed that the GitHub 'checks' on this PR are passing ✅
  (further reading)

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