refactor AlternativeImage selection logic out of Workspace into a stateless function (without downloading)

[bertsky]

we need to know whether refactoring the AlternativeImage selection logic out of Workspace.image_from_* into a stateless function (without any download_file behaviour) would break any existing API in the future, hence whether it must be done prior to 3.0 or can be done later.

@kba I don't think we need to break anything here in the future. The methods Workspace.image_from_page and Workspace.image_from_segment could be re-implemented as follows:

• delegate to new generateds user methods PageType.get_image and [*Region|TextLine|Word]Type.get_image,
  but pass as new kwarg resolve a function with the following definition:

  def resolve(image_url):
      try:
          f = next(self.mets.find_files(local_filename=str(image_url)))
          return f.local_filename
      except StopIteration:
          try:
              f = next(self.mets.find_files(url=str(image_url)))
              return self.download_file(f).local_filename
          except StopIteration:
              with download_temporary_file(image_url) as f:
                 return f.name

• replace calls to resolve_image_exif by calls to exif_from_filename directly,
  but allow overriding filename via resolve
• replace calls to resolve_image_as_pil by calls to a new function image_from_filename,
  which merely contains the parts that do Image.open() and .load() to give up the FD, as well as array conversion for the badly supported color modes I and F,
  but allow overriding filename via resolve

Originally posted by @bertsky in bertsky#21 (comment)

[bertsky]

related: #264