Correct CellMethod.__str__ method to be CF-compliant

[MoseleyS]

As shown in the CF-standard (http://cfconventions.org/Data/cf-conventions/cf-conventions-1.7/build/ch07s03.html) the correct format for writing a cell method is "coord: method". Iris currently prints "method: coord" instead which should be corrected.

[github-actions]

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[MoseleyS]

If you want something done, do it yourself...

[github-actions]

In order to maintain a backlog of relevant issues, we automatically label them as stale after 500 days of inactivity.

If this issue is still important to you, then please comment on this issue and the stale label will be removed.

Otherwise this issue will be automatically closed in 28 days time.

[trexfeathers]

Actively working on this

[MoseleyS]

Actively working on this

Thanks! I haven't got to it yet - in spite of having a reminder...

[trexfeathers]

Resolved by #5224