TST: Add veryslow option
Add pre-commit.sh
Better tests, docs, PEP8 compliance

- TST: reintroduce run_tests.sh in ./scripts/
- TST: Better test selection
- DOC: Update docs for steady_state_approx.
TST: Purge drone.io shield cache [skip ci]
Merge pull request #8 from bjodah/better_testing

Better tests.