Added a method to sort a dendrogram, and a method to return a list of lines.
Fixed bug with sorting: trunk should be sorted too
Refactored the plotting so that structures can return the lines to draw them, as individual lines or line collections, and re-implement the top-level get_leaves accordingly. Also changed the definition of height to match @ChrisBeaumont's definition and removed merge_level.
Make __repr__ show idx
Added ``Structure.get_mask`` method
Removed remaining references to ``merge_level``
Implement all the plotting in a ``DendrogramPlotter`` class - makes it easy to cache the positions of all the leaves for plotting purposes.
Added basic viewer
Import Plotter and Viewer into top-level of package
Show ID of selected structure
Implemented support for 3-d cubes, including slicing
Added sliders for vmin/vmax and prevent selection when a GUI tool is active
Set the correct vmin/vmax for the image from the start
Improved selection in 3-d slice
Ensure that the canvas is refreshed
Ensure that Matplotlib isn't needed to import ``astrodendro``
Don't include +/-inf or NaNs in min/max calculation
Added docstrings
Use grayscale colormap
Update contour when changing slice