Decide whether to keep pyqg data generators

[kysolvik]

pyqg (https://github.com/pyqg/pyqg) is no longer under active maintenance, although there has been some new discussion recently: pyqg/pyqg#330

Up until recently, the install failed without doing some special work-arounds, which made it tricky for our CI. We use pyqg for our pyqg and barotropic data generators. Do we want to:

1. Remove these generators. We would still have pyqg-jax, although that's trickier to work with than regular pyqg.
2. Keep and update them. They need to be updated to output xarray objects to match the other data generators.