Improve db benchmarks with pytest

[alecandido]

New pytest setup:

• move ParentTest in a conftest.py to make it an autoimported fixture
  • give it a better name, both to class and the instances inside the test functions
• insert some test levels, such that everything can be run through pytest
  • we can use custom marks: pytest.mark.<custom>
    • see also how to configure to run
  • put a 'workflbow level' for running on GitHub
  • put a 'quicktest level' or 'local level' for running locally
• enrich ParentTest API
  • add a small navigator to show thoeries/observables, cache/logs
    • we can also present them in a pager, there is a python module able to output content in a pager, as easy as using print() function
    • list all table entries in a quick form (just the id and relevant fields/comment), pretty print content
    • maybe add quick query support, maybe not

Not a priority

• add test dependencies in setup.py
  • update workflows consistently
• separate commit workflow from merge workflow

[felixhekhorn]

closes #39