input: Write a python function to calculate the distance of a Point3D object from the origin.
  assert Point3D(3, 4, 0).distance_from_origin() == 5.0
  assert Point3D(0, 0, 0).distance_from_origin() == 0.0
  assert Point3D(1, 1, 1).distance_from_origin() == (3**(1/2))
output: import math
  
  class Point3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def distance_from_origin(self):
          return math.sqrt(self.x**2 + self.y**2 + self.z**2)