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

首先检查 #2175 中的兼容性要求，以及 #2799 和 #3300 中链接的讨论。然后跟踪现有的 multicategory-axis 行为以及相关的 bargap 和 bargroupgap 选项；当能够控制外层间距，同时不破坏 ticks、gridlines 或现有的间距行为时，即可视为完成。