sgkit.genee improvements

[jdstamp]

• Improves documentation of input and output for sgkit.genee
• The Example for the sgkit.genee doc string is not a final version. It would need a better treatment of the LD matrix situation to be useful. The major point of this suggestion is to start a discussion about how to make the method more usable and also have an example that explains a user how to provide windows for regions of interest.
• Changing the example code to have more contigs and overlapping windows in different contigs produces some edge cases that are not handled by the method.
• How can the test_genee.py file be made active during unit tests? I was only able to run it specifically using pytest -v path/to/file. I added the path to the data default because I don't know how to properly configure that as part of the testing framework.
• Accompanies genee review: sgkit implementation limitations #1180

[tomwhite]

These look good to me. Thanks @jdstamp!