splits_atts with test against human genome GFF

[joelb123]

The pull incorporates the split_atts code that fixes reading of some GFFs along with some improvements to testing:

• rather confusing set of symlinks removed
• test files reformatted with black
• each test takes place isolated in its own tmp_dir
• human genome GFF is downloaded and used as a simple test, printing out timing for performance
• travis build is fixed (I think), with cache of human GFF and repositories turned on

[joelb123]

a problem showed up with the travis build, submitting a new PR

[joelb123]

Fix is done, Travis builds work for me.