Add https://snap.berkeley.edu/ in the allowed iframe site setting and then you will be able to embed the URL into an iframe on posts:
<iframe src="https://snap.berkeley.edu/project?user=bh&project=py-fractal"></iframe>
You should also set width and height and other parameters
<iframe src="https://snap.berkeley.edu/project?user=bh&project=py-fractal" width="xxx" height="xxx"></iframe>