Integrate non-linear dim. reduction techniques

[raamana]

So far the dimensionality reduction techniques offered are only feature selection methods, and feature transformation methods.

it's about time to integrate non-linear (and linear) dimensionality reduction techniques, to offer maximum choice (and potential benefit) to users: https://scikit-learn.org/stable/modules/manifold.html#manifold

[raamana]

#44 addresses this.