input: Write a python function to calculate the magnitude of a 3D vector.
  assert Vector3D(3, 4, 0).magnitude() == 5.0
  assert Vector3D(1, 1, 1).magnitude() == (3**(1/2))
  assert Vector3D(0, 0, 0).magnitude() == 0.0
output: import math
  
  class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def magnitude(self):
          return math.sqrt(self.x**2 + self.y**2 + self.z**2)