Make BasicDendrogramViewer extensible about astrodendro

[astrofrog]

@tomr-stargazer - I'm all for making the DendrogramViewer more flexible! To some extent, we need to make sure that we don't reinvent Glue (cc @ChrisBeaumont) so it might be worth seeing how much the code here can be extended, versus how much Glue could be used instead. If you can show what you were thinking, it might help understand how much overlap there is with Glue.

[tomr-stargazer]

Thanks for the quick feedback! I agree that Glue is definitely worth looking into; I haven't used it yet even though I've seen and heard lots about it.

Among the upgrades I'm hoping for:

• Multiple panels showing corresponding data (as mentioned above, e.g. both an (l, b) and an (l, v) image, see this image for a crude mockup)
• option to show integrated images instead of slices (I just submitted this as Issue #95)
• The ability to select multiple branches in different colors and see their corresponding regions, as was possible in the old IDL code (e.g. this screencap ) - might submit this as a separate issue if needed

[ChrisBeaumont]

Since there is ~no dendrogram support in Glue yet, I am definitely in favor of figuring out the important features for interactive analysis within this package, and then porting over to Glue (for the cross-file support and more robust UI stuff). We should keep the Glue use-case in mind when adding new features, to make it easier to port

I think multiple selections and #95 would be more straightforward to implement than the multiple image panels thing, so I would start there first. Of course, Glue will solve the multiple image panel thing automatically