Support the elliptical arc command for SVGMobject

[YishiMichael]

Motivation

Support the elliptical arc command for SVGMobject. Now all the commands of svg path can be implemented.

Proposed changes

This is reached through approximating elliptical arcs with quadratic bezier curves, which is almost the same way to form a circular arc in Arc class. The key is to figure out the center of the ellipse, requiring us to solve simultaneous equations. I've built a function add_elliptical_arc_to(self, rx, ry, x_axis_rotation, large_arc_flag, sweep_flag, point), inside which a bunch of functions were defined just to do those calculations. I guess it might be better to transfer them into the file utils\space_ops.py, but I daren't make too much changes across files in my first pull request.
Detailed annotations are left in the code. I deduced all those stuff by hand today .(:з」∠).

Test

Code:

class Test(Scene):
    def construct(self):
        self.add(SVGMobject("test"))

<svg width="320px" height="320px" version="1.1" xmlns="http://www.w3.org/2000/svg">
  <path d="M10 315
           L 110 215
           A 30 50 0 0 1 162.55 162.45
           L 172.55 152.45
           A 30 50 -45 0 1 215.1 109.9
           L 315 10"/>
</svg>

Result: