calculate subcortical activation

Adds an example to examples/inverse using the spm faces dataset. Calculates the activation for a subcortical volume. 
Instead of constructing `X` as the combination above, I would construct `s1` and `s2` separately, then you can just do `X = np.concatenate((s1, s2), axis=0)`. This will be clearer / fewer lines. In fact, I don't think you'll even need `X` at all, since every time it's used it's split anyway...