Hotfix to improve GFR numerical stability when adjacency is nearly zero.

Many updates since 0.7

- Fixed a QM9 downloader issue.

- Fixed a bug with the Maximin metric when some hyperparameters are fixed.

- Downloader for QM9.

fix unit test

- A new and experimental calling convention to allow evaluations of the

  graph kernels at a list of specific indices.

- A sequence-based rewriter for Monte Carlo tree search.

- Convert any kernel into a metric via `KernelInducedDistance`.
- Convert any norm into a kernel via `KernelOverMetric`.

- Improvements to the active learning hierarchical drafter.

- Performance optimization for GFR leave-one-out cross validation gra…

…dients.

- Leave-one-out cross validation for Gaussian field regressor.