Tidy iris.experimental module

[bjlittle]

Identify and pull through any experimental functionality from iris.experimental into the core of iris, and also retire any obsolete functionality.

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Acceptance Criteria

• Remove iris.experimental.concatenate (see Remove iris.experimental.concatenate module #3523)
• Rationalise iris.experimental.equalise_cubes (see Move experimental equalise_attributes #3527)
  • Move iris.experimental.equalise_cubes.equalise_attributes to iris.util
  • Replace iris.experimental.equalise_cubes.equalise_attributes with an exception, e.g., see here
• Review outstanding iris.experimental functionality
• NB: Must include identify all that are used operationally, (def includes areaweighted regridding). as they must not be deprecated.

[bjlittle]

Ping @abooton