class_docstr: Build a Sliding object.
  
  A window starting at zero and going to some point that increases between windows.
  
  Parameters
  ----------
  length: int, The length of the window.
  step: int, The sliding step size.
code: def __init__(self, length, step):
      super().__init__(
          initial_length=length, start_step=step, end_step=step
      )
  
imports: import collections
  import numpy
label_desc: Use image node_id 1 to create a new _Sliding object from inherited base classes: _ExpandingSliding with example: obj = _Sliding(length, step)