manifolds.Sphere: Add barycentric coordinates of spherical triangles / Whitney forms

[mkoeppe]

On S², given three generic points, we define a chart of barycentric coordinates for the spherical triangle; see for example
https://arxiv.org/abs/1404.6592

CC: @mjungmath @egourgoulhon

Component: manifolds

Issue created by migration from https://trac.sagemath.org/ticket/31777