Implement the solid angle measure (the plane angle) of a two-dimensional cone, using the dot-product (trigonometry) formula.

For example, the solid angle of the first orthant solid_angle_simplicial_2d([[0,1],[1,0]])
is 1/4.

Later we will add methods for three-dimensional and higher-dimensional cones.

CC: @yuan-zhou @mkoeppe

Component: geometry

Keywords: solid angle

Author: Allison Fitisone, Yuan Zhou

Branch/Commit: u/yzh/implement_solid_angle_measure_for_two_dimensional_simplicial_cones @ d8a5f5a

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/33656