Add statistics to archives

[btjanaka]

Description

This PR adds various attributes that provide statistics about an archive.

Resolves #33

Changelog Description

Add statistics attributes to archives (#100)

TODO

• Add and test attributes - qd_score, obj_max, obj_min, obj_mean, obj_median, coverage
  • obj_min and obj_median are left out because they require more computational effort (we would either add a SortedList or use O(n) or O(n log n) brute force). Furthermore, these metrics are not very common in the literature. Note that the metrics can still be computed from archive.objective_values.
• Use attributes in tutorials -> followup (see Update examples and tutorials with latest API changes #148)

Questions

Status

• I have read the guidelines in
  CONTRIBUTING.md.
• I have formatted my code using yapf
• I have tested my code by running pytest (tox is run in CI/CD)
• I have linted my code with pylint
• This PR is ready to go