[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 is the difference between plot_ica_resting_state_.py (with an underscore)
> and plot_ica_resting_state.py (without an underscore)?

Looks like a bug to me.