Tangent space embedding

Different connectivity measures, including tangent space measure
I would go for a `plot_connectivity_matrix` function that set a good colormap and do fancy labelling (see seaborn[2]). We could even propose to rearrange labels by block (using a spectral clustering for exemple) and give a different color for each block (see bokeh [1]).

[1] http://bokeh.pydata.org/docs/gallery/les_mis.html
[2] http://stanford.edu/~mwaskom/software/seaborn/examples/many_pairwise_correlations.html