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

Important

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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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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.42%. Comparing base (3b4089e) to head (c42ccbb).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7236      +/-   ##
==========================================
+ Coverage   90.34%   90.42%   +0.07%     
==========================================
  Files          92       93       +1     
  Lines       25336    25814     +478     
  Branches     4731     4796      +65     
==========================================
+ Hits        22890    23342     +452     
- Misses       1675     1693      +18     
- Partials      771      779       +8     

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