push hover labels in to the bar group extent in compare mode

[alexcjohnson]

Fixes #2350

Adds an extra step to Bar.setPositions to record the position extent of each bar group, which includes two loops over the data: one to find the min and max position of the trace group, and a second to calculate the extents. I did it this way because calculating this on the fly in hoverPoints is fairly awkward, you either have to loop over traces once for each trace (repetitious) or you could modify the hover data after finishing hoverPoints, manipulating all the bar traces in the hover data to match their position axis coordinates (adding type-dependent logic to Fx.hover directly)... this way seemed cleaner. There may be ways to move some of these calculations into existing loops in setPositions but the way these are set up this seems like it would entail a lot of duplication, and anyway I don't expect this to be a noticeable overhead for bars.

cc @etpinard

[etpinard]

👍 for putting the hard parts in setPositions as opposed to beefing up hoverPoints.

💃 💃 🗡️ for your third bug fix of the day.