Do not assign offsetgroup to traces in px unless barmode="group"

[emilykl]

Closes #4863

This change is required due to the change in behavior for offsetgroup introduced in Plotly.js 3.0: plotly/plotly.js#7009

The above change causes bars to appear as a group if offsetgroup is set, even if barmode is set to stacked, which changes the default behavior of histograms and bar charts, as explained in #4863 (see comment).

This PR fixes the issue by only setting offsetgroup if barmode="group". (We also set offsetgroup if barmode is not set, since group is the default barmode in Plotly.js.)

Note that we can't remove setting offsetgroup entirely due to the issue described in my comment here.

The other change (changing the point at which we set barmode to overlay if the figure has marginal histograms) is so that the figure's barmode in that case will be set before the point where we decide whether to set offsetgroup.

[emilykl]

Unfortunately, not using offsetgroup at all breaks an edge case for faceted charts, as demonstrated by the percy tests histogram_histfunc and bar_facet.

Without offsetgroup, when there is no data in a facet for one bar in a group of bars, the other bars expand to fill the available space.

See comparison below — I believe the behavior on master is the way that this chart should be rendered.

master	fix-bar-charts (this branch)

[emilykl]

Note: The 4 visual changes in Percy are the 4 charts which were incorrect on master and are now returning to how they should look.

[gvwilson]

thanks @emilykl - @marthacryan please review

[marthacryan]

LGTM