code: def sphere(target, pore_diameter='pore.diameter'):
      r
      diams = target[pore_diameter]
      value = _pi/6*diams**3
      return value
docs: r"""
      Calculate pore volume from diameter assuming a spherical pore body
  
      Parameters
      ----------
      target : OpenPNM Object
          The object which this model is associated with. This controls
          the length of the calculated array, and also provides access to other
          necessary geometric properties.
  
      pore_diameter : string
          The dictionary key of the pore diameter values