Diffusion distance

TODO. Will cover:

• Heat-kernel rows $P^t(u, \cdot)$ as multi-scale neighbourhoods.
• $d_{\mathrm{diff}}^t(u, v) = \| P^t(u, \cdot) - P^t(v, \cdot) \|_{\ell^2(\pi^{-1})}$.
• Limit behaviour as $t \to \infty$.
• IsHypergraphMetric instance status.