Add Tadpole graph

[ghost]

Implements the Tadpole graph
Source: https://en.wikipedia.org/wiki/Tadpole_graph

[dschult]

This isn't hard to construct from the others, but it does seem to be referred to regularly in the literature and might be common enough to warrant inclusion. Some comments appear below.

[dschult]

We have a question as to whether we should allow a tadpole to have m=0 (no head to the tadpole). I'm +0 for allowing that. It differs from the treatment for lollipop_graph where the head must contain at least 2 nodes (to be a complete graph). Here we have a cycle connected to the path, and a cycle can have a single node (with a self-loop) or no nodes.

I have a few suggestions below and otherwise think this is ready to merge.
Thanks!

[ghost]

I probably clicked a wrong button, because I cannot see my own reply, but here's my opinion;

Using m=1 should, IMO, give a single node without links. @dschult suggested a single node with a self-loop. One possible way to avoid these ambiguities is to enforce m >= 2. But my preference would be to have m=1 and general n to return a path graph with n+1 nodes.

I also don't understand what's wrong with the CI

[ghost]

Changed to throw an error when m<2. Still cannot find out why CI fails.

[dschult]

To fix CI, merge the upstream main with your branch.
Steps are hopefully something like:

git remote add upstream https://github.com/networkx/networkx
git fetch upstream main
git ch Add-tadpole-graph   # if needed
git pull upstream main --rebase

The last line will try to apply your commits since you diverged from main on top of the current main branch.
If you have changed a line that was also changed on main you will get a "conflict" and you'll have to follow the instructions to edit the file by hand to specify which changes you want to keep. That can be a mess sometimes.
Can you try it and see if everything "just works"? If it doesn't, I can work through that. Thanks.

The last line can also be: git merge upstream main which I believe is the same as git pull upstream main --no-rebase and that can sometimes be easier to deal with the conflicts. But it adds an extra commit if I remember the details right.

I think the conflicts are also on another of your PRs. I have on my list to work through those conflicts. :}

[dschult]

This looks ready to me!