Remove ensemble loading heuristics

[jfrost-mo]

We now rely on ensemble data having a realization coordinate.
This allows for a significant simplification of the loading process.

Fixes #1808
Fixes #1845

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[github-actions]

[jfrost-mo]

#1872 implements a similar fix to 7cb7d26 as part of a larger set of changes. Note that it will probably conflict.

[SGallagherMet]

@mo-tomosevans could you comment on this? I know you've been testing ensemble data
My gut feeling is that this will break out trial and operational data as the 'realisation' number is not included in the cube metadata for the control member so has to be inferred from the file name.
That being said, I'm not sure the code that has been removed would have worked anyway, as the operational and trial file naming conventions are different.

[mo-tomosevans]

Hi

I think this is PR is something that could address problem I've encountered in #1845. Someone else reported something similar in #1808.

You're right though @SGallagherMet - in the ensemble trial data I've been testing, the ensemble members are given as realization coordinates in the cubes for all but the control member (i.e. <path>/enuk_um_000/enukaa_pd*). In this case there is no realization coordinate by default.

[jfrost-mo]

the 'realisation' number is not included in the cube metadata for the control member

Am I right in thinking the control would be member 0? If so then it would have a realization coordinate added by the _realization_callback, which should then cause it to merge into a single cube with the other members (assuming I've picked the right coordinate type, etc.)

Is there any operational/trial ensemble data on disk that I could give this a test with?