class_docstr: A :class:`RasterSource` which wraps another, an then applies a
  post-processing step on the raster fetched from the contained source.
code: def fetch_raster(self, *args, **kwargs):
      fetch_raster = super().fetch_raster
      located_imgs = fetch_raster(*args, **kwargs)
      if located_imgs:
          located_imgs = [
              self._post_fetch_fn(img) for img in located_imgs
          ]
      return located_imgs
  
imports: import collections
  from pathlib import Path
  import string
  from urllib.request import urlopen
  import warnings
  from cartopy import config
label_desc: Use image node_id 2 for calling the PostprocessedRasterSource obj's underlying member method code with example usage: obj.fetch_raster() and returns: located_imgs