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 std(self):
      return np.sqrt(self.variance)
  
imports: import numpy
label_desc: Use image node_id 5 for calling the OnlineMoments obj's underlying member method code with example usage: obj.std() and returns: np