Coordinate system agnostic region extraction

[david-bentley]

Description

Add a function to extract a region, decoupling the specified region bounds from the coordinate system of the cube. For example, given a cube in a rotated pole coordinate system

>>> print(cube)
air_pressure / (Pa)                 (grid_latitude: 151; grid_longitude: 97)
    Dimension coordinates:
        grid_latitude                             x                    -
        grid_longitude                            -                    x
    Scalar coordinates:
        forecast_reference_time     2013-08-15 03:00:00
        level_height                5.0 m
        sigma                       0.9994238, bound=(1.0, 0.99846387)
        time                        2013-08-15 11:00:00
    Attributes:
        Conventions                 'CF-1.7'
        STASH                       m01s00i408
        source                      'Data from Met Office Unified Model'
        um_version                  '8.2'

one can specify a subregion in a regular lat/lon coordinate system to extract that region from the rotated pole cube:

>>> area = (-4.790656510485121, 50.14530257322982, -3.450313532058675, 51.72890111361691)
>>> print(iris.util.extract_region(cube, area, crs=ccrs.PlateCarree()))
air_pressure / (Pa)                 (grid_latitude: 120; grid_longitude: 66)
    Dimension coordinates:
        grid_latitude                             x                    -
        grid_longitude                            -                    x
    Scalar coordinates:
        forecast_reference_time     2013-08-15 03:00:00
        level_height                5.0 m
        sigma                       0.9994238, bound=(1.0, 0.99846387)
        time                        2013-08-15 11:00:00
    Attributes:
        Conventions                 'CF-1.7'
        STASH                       m01s00i408
        source                      'Data from Met Office Unified Model'
        um_version                  '8.2'

The function accepts the same keyword arguments as cube.intersection() (e.g. ignore_bounds, threshold).

Closes #6855

Checklist

• 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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