- New class ixmp.config.Config to regularize config handling.
- Remove ixmp.default_paths, ixmp.default_path_constants.
- cli.config(): use Config.save().
- Scenario__init__(): use Config.find_dbprops() and Config.get().
- conftest.py: new fixture 'tmp_env' and hook 'pytest_sessionstart'.
- test_config.py: test Config features.
- test_core.py: use pytest 'caplog' fixture to test that configuration
  values are used.
- test_tutorials.py: use 'tmp_env' to force tutorial execution in a
  sandbox.