Analysis and Visualisation

[Christovis]

This PR adds:

• new analysis functions:

  class ListservList:
  
  Methods
  -------
  classmethod:
      from_mbox
      from_pandas_dataframe
  staticmethod:
      to_percentage
      contract
      get_name_localpart_domain
      iterator_name_localpart_domain
  period_of_activity
  crop_by_year
  crop_by_address
  get_domains
  get_domainscount
  get_localparts
  get_localpartscount
  get_messagecount
  get_messagecount_per_timezone
  get_sender_receiver_dictionary
  create_sender_receiver_digraph
  get_graph_prop_per_domain_per_year

• new visualisation functions:

  evolution_of_participation
  evolution_of_graph_property_by_domain
  stacked_area_chart