Add new tax function: monotone cubic spline regression

[rickecon]

This issue is relevant to three other open issues (#493, #574, #699). We are proposing to add a new tax function estimation method, which is a monotonically nondecreasing cubic spline regression. This approach will allow for smooth transitions at kink points (similar to the 0% tax thresholds described in Issue #493) and for points of inflection caused by increased taxes at the high end of the income distribution (see Biden campaign tax proposal analysis here).

I am posting my work on this in this Google Colab notebook until I officially open a PR.

cc: @jdebacker

[rickecon]

Here is a picture of the nonmonotonic cubic spline regression (knot points at $50k, $100k, $500k, and $1m). Code is in the Colab Notebook.

[rickecon]

This issue will be resolved and closed when PR #819 is merged.