Support control over gaps between outer layers of multicategory axes

Adding additional whitespace padding between outer layers of a multicategory axis can improve the legibility of visualizations in many use cases, particularly when the emphasis is on making comparisons within outer-layer categories or between outer-layer categories (not just across all units).

Should be compatible with #2175.

Original discussion of such a feature:

Indeed, I've been pondering if there's a way to improve that. Certainly showing gridlines for the outer category only would help. Trying to think through an analog of bargap / bargroupgap, ie to give a bigger gap at the outer category boundary... but it seems tough to square that with the ticks & gridlines. Also would probably be a major headache to implement, as the spacing between categories would no longer be constant.

Originally posted by @alexcjohnson in https://github.com/plotly/plotly.js/issues/2799#issuecomment-442243851

Hi is there support for something similar to bargap and bargroupgap with multicategory axes? The goal would be to have a different gaps between bars of an outer category and bars of an inner category. Thanks ! Any other way to achieve this would be helpful :)

Originally posted by @elmadj in https://github.com/plotly/plotly.js/pull/3300#issuecomment-575856796

Inizia esaminando il requisito di compatibilità in #2175 e le discussioni collegate in #2799 e #3300. Poi traccia il comportamento esistente di multicategory-axis e le opzioni correlate bargap e bargroupgap; il lavoro è completo quando gli spazi del livello esterno possono essere controllati senza compromettere i ticks, le gridlines o il comportamento di spaziatura esistente.