Return the angle between two points, which is also equal to the distance
between these points on the unit sphere. The points do not need to be
normalized. This function has a maximum error of 3.25 * DBL_EPSILON (or
2.5 * DBL_EPSILON for angles up to 1 radian). If either point is
zero-length (e.g. an uninitialized S2Point), or almost zero-length, the
resulting angle will be zero.
Return the angle between two points, which is also equal to the distance between these points on the unit sphere. The points do not need to be normalized. This function has a maximum error of 3.25 * DBL_EPSILON (or 2.5 * DBL_EPSILON for angles up to 1 radian). If either point is zero-length (e.g. an uninitialized S2Point), or almost zero-length, the resulting angle will be zero.