GroupTable presentation like in XL

Hi

When there is only 1 line in the last group of the hierarchy it is unnecessary to create a group line, this pollutes reading of figures.

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E.g in XL you will have something more easy to read

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Also, it would be nice to allow a style provider specific to group lines to give them a specific style. I managed to increase font weight for group names, but for not for aggregated figures as they all share the same style.

Regards
Michael