class_docstr: Sorting key for all galleries subsections
  
  This requires all folders to be listed otherwise an exception is raised
  
  Parameters
  ----------
  ordered_list : list, tuple, types.GeneratorType
      Hold the paths of each galleries' subsections
  
  Raises
  ------
  ValueError
      Wrong input type or Subgallery path missing
code: def __call__(self, item):
      if item in self.ordered_list:
          return self.ordered_list.index(item)
      else:
          raise ValueError(
              "If you use an explicit folder ordering, you "
              "must specify all folders. Explicit order not "
              "found for {}".format(item)
          )
  
imports: from __future__ import division, absolute_import, print_function
  import os
  import types
label_desc: Use image node_id 2 for calling the ExplicitOrder obj's underlying member method code with example usage: obj.__call__(item) and returns: self