SQGTurb debugging and various other fixes

[kysolvik]

Started out as fixes to make sqgturb work, but that uncovered some other issues that I fixed along the way. In all:

• sqgturb and pyqg_jax now produce xarray datasets (fixes #57)
• Enables nearest-neighbor observation selection
• Fixes observation sampling for multi-dimensional dataset
• Makes DA cycler work with multi-variable datasets (see _rebuild_dataset() method in _dacycler.py)
• Adds hybrid gain DA method
• Various other fixes, mostly to make observers and da cyclers work better for datasets with different dimensions, number of variables, etc.