Review and update example code in README

[richarda23]

• add some examples of Inventory API
• make sure all snippets work with v2

[burgerga]

I was trying out some examples from the README:

• https://github.com/rspace-os/rspace-client-python#a-basic-query-to-list-documents

  AttributeError: module 'rspace_client' has no attribute 'Client'

  Should probably be ELNClient

• https://github.com/rspace-os/rspace-client-python#getting-attached-files

  Doesn't work for me; long 404 error on the line

  download_metadata_link = client.get_link_contents(file, 'self')

  What does work is

  document_id = 17828 #< replace with own doc_id
  try:
      response = client.get_document(doc_id = document_id)
  except ValueError:
      print('Document with id %s not found' % str(document_id))
      
  for field in response['fields']:
      for file in field['files']:
          filename = '/tmp/' + file['name']
          print('Downloading to file', filename)
          client.download_link_to_file(client.get_link(file, 'enclosure'), filename)