class_docstr: Compute 1st and 2nd moments online (to avoid storing all elements).
  
  See algorithm at: https://www.wikiwand.com/en/Algorithms_for_calculating_variance#/Online_algorithm
code: def __init__(self):
      self._mean = 0
      self._M2 = 0
      self._n = 0
  
imports: import numpy
label_desc: Use image node_id 1 to create a new OnlineMoments object from inherited base classes: object with example: obj = OnlineMoments()