Initialize Fitters by variance or n_components

This PR allows `LucasKanadeAAMFitter` and `GradientDescentCLMFitter` to be initialized in terms of the variance ratio capture by the shape and appearance eigenvectors. Combined lists with number of components and and variance ratio are also accepted.

    fitter = LucasKanadeAAMFitter(aam, algorithm=AlternatingInverseCompositional, n_shape=[3, 0.95], n_appearance=[0.3, 20])