Add pyyaml as core dependency

[cdeil]

This PR adds pyyaml as a core dependency of Gammapy.

In practice this means that we list pyyaml in install_requires in setup.py, and that pip install gammapy will always install pyyaml automatically as a dependency if it isn't already present.

We currently mostly use YAML to read ECSV files, as well as for the gammapy download functionality. We already use it for model serialisation a bit, and plan to use it more extensively in the future (xref #329). Very likely YAML will be used in the high-level Gammapy interface as config file format (and for model serialisation), so really the vast majority of users will need it. Fermipy does already use it, so this is not adding an extra dependency for them.

The changes here are similar to what @adonath did when we made Scipy a core dependency in #1919 - mostly moving imports to the top and removing a few requires_dependency decorators on tests. I plan to mention this clearly in the changelog, but in a follow-up PR.

[adonath]

Looks good to me, I have no further comments.

[Bultako]

👍