input: Write a function to determine if a given 3-dimensional vector is a unit vector, meaning its length is equal to 1. Create a function named `is_unit_vector` that takes an instance of the `Vector3D` class as input and returns True if the length of the vector is 1, otherwise returns False.
  
  
  assert is_unit_vector(Vector3D(1, 0, 0))==True
  assert is_unit_vector(Vector3D(0, 1, 0))==True
  assert is_unit_vector(Vector3D(0, 0, 1))==True
  assert is_unit_vector(Vector3D(1, 2, 3))==False
  assert is_unit_vector(Vector3D(-1, -1, -1))==False
  
output: def is_unit_vector(vector):
      return vector.x**2 + vector.y**2 + vector.z**2 == 1