Allow users to adjust amount of separation between first barplot layer and the farthest tip

[fedarko]

Brought up by @gwarmstrong at this morning's meeting.

Having the ultrametric PR in, as well as #366 and #308 in, definitely reduces the need for this -- but if demand for this remains in the future we may want to work on it.

For reference, a solution to this issue would involve adding an extra <input type="number"> to the barplot section of the side panel that allows users to configure the 1.1 value listed here (or something similar -- there is probably a more user-friendly way of exposing this functionality):

empress/empress/support_files/js/empress.js

Lines 1490 to 1499 in ae42a08

	// Add on a gap between the closest-to-the-root point at which we can
	// start drawing barplots, and the first barplot layer. This could be
	// made into a barplot-panel-level configurable thing if desired.
	// Currently, the 1.1 term here means that the barplots start at the
	// max displacement plus 1/10th of the max displacement. If we used
	// a 1.0 term instead, then barplots would start immediately at the max
	// displacement (this looks kinda bad because the node circle of the
	// tip(s) at this max displacement are partially covered by the
	// barplots, so we don't do that).
	var maxD = 1.1 * this._maxDisplacement;