Utility to enforce coordinate order

[gavinevans]

As an IMPROVER developer, I want a single utility that consolidates existing functionality that orders coordinates so that I can easily pass a cube into the utility and be confident that the resulting cube has coordinates in the correct order.

Related issues: #209, #166, #236

Existing utilities:

• Investigate the make_stat_coordinate_first method in #236.
• Investigate the ensure_dimension_is_the_zeroth_dimension function within ensemble_calibration_utilities.py
• Investigate check_cube_coordinates method.

Things to consider:

• Order of preference for realization, percentiles, thresholds. Note that there can be both realizations and percentiles on a cube.
• Vertical coordinates: height, pressure.
• Time.
• Equal preference for projection_x_coordinate, longitude and grid_longitude.
• Equal preference for projection_y_coordinate, latitude and grid_latitude.

This utility will be extended in the future, as use cases emerge.

Acceptance criteria:

• Make sure that the coordinate ordering follows the agreed temporary conventions for probabilistic data.
• Identify use cases for the existing utilities and identify whether this use case can be served by a single utility or that there is a requirement for multiple utilities.
• Consolidate existing utilities, if possible.
• Unit tests.

[gavinevans]

Use case: In circular_kernel.py, there is a section of code that reorders the resulting cube. This could be factored out and potentially generalised, as at the moment, it is hard-coded to use the percentiles_over_neighbourhood coordinate. There is some similar functionality existing in check_cube_coordinates for modifying coordinates within an output cube to match the coordinates within the input cube. However, the function suggested in this issue would order the coordinates within an output cube without the need for an input cube. See discusstion in https://github.com/metoppv/improver/pull/259#discussion_r143487493.

[bayliffe]

Work completed.