Make Ocellaris compatible with PyYAML 5.1

[Tormod Landet]

PyYAML 5.1 does not respect dict ordering(?), breaking a unit test. See commit d6fd9cd

To fix:

• Make Ocellaris + PyYAML 5.1 not reorder keys so that the 'tests/test_io.py::test_restart_file_io' passes also with the new version of PyYAML.
• update yaml.load to take a Loader= param, see PyYAML release notes or the wiki article.

[Tormod Landet]

Fixed by pull request #22