Add option to set dendrogram color threshold

[paulamool]

Hope this works @Elpiro @jonmmease

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Adds a new optional color_threshold argument to the create_dendrogram function to customize the dendrogram "cut height" at which clusters are separated.

This work was initially started by @Elpiro in #1075

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Examples

import plotly.graph_objs as go
import plotly.figure_factory as ff
import numpy as np

X = np.random.rand(15, 15)

Default behavior

dendro = ff.create_dendrogram(X)
fig = go.FigureWidget(dendro)
fig

With a color threshold

dendro = ff.create_dendrogram(X, color_threshold=1.6)
fig = go.FigureWidget(dendro)
fig

[paulamool]

Related to pull request #1075 Added parameter to choose color_threshold and issue #995

[jonmmease]

Thanks @paulamool and @Elpiro, I'll take a look in detail soon, but this looks like a great addition and we should be able to get this into 3.4.0.

[jonmmease]

Looks good and works well. Merging. Thanks for the work @Elpiro and thanks @paulamool for pushing it over the finish line!