estimate_normal: Move to plane, test, and refactor

[paulmelnikow]

• Add tests (ref 100% test coverage #52)
• Add documentation
• Use vg.major_axis, avoiding use of undeclared scipy dependency
• Use vg.shape.check

[paulmelnikow]

This function is very similar to Plane.fit_from_points():

polliwog/polliwog/plane/plane.py

Lines 69 to 79 in 8141e66

	@classmethod
	def fit_from_points(cls, points):
	"""
	Fits a plane whose normal is orthgonal to the first two principal axes
	of variation in the data and centered on the points' centroid.
	"""
	vg.shape.check(locals(), "points", (-1, 3))
	eigval, eigvec = np.linalg.eig(np.cov(points.T))
	ordering = np.argsort(eigval)[::-1]
	normal = np.cross(eigvec[:, ordering[0]], eigvec[:, ordering[1]])
	return cls(points.mean(axis=0), normal)