Describe the Enhancement

#1815 enhanced MET-11.0.0-beta2 to support the specification of NetCDF level strings with the VALUE of the dimensions rather the just the INDEX of the dimensions. @robdarvell tested out this new functionality in beta2 and provided feedback to refine the functionality.

For a variable with a dimension that contains 24 levels, he requested level='(@23,*,*)';. He expected this call to fail since the corresponding coordinate variable does NOT contain a value of 23. However running plot_data_plane in this way ran without error and it just used 23 as the INDEX for that dimension.

Recommend that we tweak the logic for parsing these levels strings. If the sting begins with an '@' character always interpret it as a VALUE and never as an INDEX.

It is not clear to me how the converse should be handled. For simplicity, should the lack of an '@' character force the interpretation to be as an INDEX and not a VALUE? The issue here is the time dimension. MET version 10.1.0 allows for the time dimension to be specified as a formatted string. For for this NetCDF variable data(time,lat,lon) the level='(20220807_120000,*,*)'; string correctly retrieves the data. In MET 11.0.0-beta2, level='(@20220807_120000,*,*)'; now also works.

@hsoh-u can we make the presence of the leading @ character optional for time strings to preserve backward compatibility?
But make their presence required when specifying values for non-time strings?

@robdarvell any opinion on this detail?

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