[WIP] Add an example using spectral clustering

This PR is WIP and is not yet ready to merge.

I added an example using single subject spectral clustering. This is a very simple example. I will also use this PR to migrate other examples to ADHD dataset (instead of NYU).
What about simplifying using:

```python
values[i] = 0. if np.isnan(corr) else corr
```

instead of the if + continue.