Add {Plane,Cone}.from_points() methods

[egpbos]

Inspired by the NormalDist.from_samples() syntax, I think it would be nice if the shape classes could have constructors that take a point set as input, fit to that point set, and use the best fitting parameters to make the new shape. It would look like this:

from numpy.random import random, normal
from ectopylasm import Plane

N = 1000
xyz = (random(N), random(N), normal(0, 0.01, N))

plane = Plane.from_points(xyz)