sage/topcom: add an explainer somewhere in a "howto" section

[haraldschilly]

Explain what's going on with "TOPCOM". I'm not able to install it in recent Sage versions, but 9.5 to 9.7 in Ubuntu 20.04 seem to work. Beyond that, I don't know anything about this package regarding its functionality.

Some text about this:

TOPCOM doesn't work with recent versions of Sage – at least in the environment CoCalc provides. But, CoCalc let's you can explore the history of older software. In particular, the Sage 9.5 we provide seems to have a working TOPCOM under the hood. To access it:

• In project settings → project control → software environment: switch to "Ubuntu 20.04 // Deprecated" (it's the 2nd at the top).
• In a jupyter notebook select the "Sage 9.5" kernel. It might be that newer/older versions of Sage have it installed as well, but I didn't check. 9.5 was just the first one I tried. 9.6 or 9.7 might work as well.

Below is a screenshot, where I copy/pasted some lines from the sage docs for topcom. It even spit out a 2d and 3d plot.

[trevorkarn]

I just tried to use it on Cocalc with both Ubuntu 22.04 and 20.04 running Sage 9.6, 9.7, 9.8. It worked with Ubuntu 20.04 and Sage 9.6, 9.7, but neither of them worked with 22.04 and also it didn't work with 9.8. I tested by running the following example in a jupyter notebook.

PointConfiguration.set_engine('topcom')
p = PointConfiguration([[-1,-5/9], [0,10/9], [1,-5/9],
                        [-2,-10/9], [0,20/9], [2,-10/9]])
p_regular = p.restrict_to_regular_triangulations(True)