Function normal_lines to ellipse.py

I browsed through the ellipse class and found out that there was no method for returning normal lines.So I wrote a simple function which returns normal lines to the ellipse.

- [x] add test case for 3 roots (Fig 2.3 in paper on ellipse normals from http://www.internationalmathematicasymposium.org/IMS99/ims99papers/ims99papers.html)
- [x] add test case for point on ellipse