improve gff: add Locatable to Gff3BaseData, add 'Optional<String> getAttr(key)' and 'hasAttribute'

[lindenb]

Description

This PR improves the GFF3 classes:

• 99% the interesting attributes in a GFF3 file are unique (for example gene_biotype, gene_name, etc...) , so using the function List<String> getAttribute(key) can be cumbersome. This PR adds a new function Optional<String> getAttr(key) , it returns an Optional if there is zero or one value. Otherwise an exception is thrown.
• I also added a function hasAttribute to test for the presence of an attribute.
• add implements Locatable to Gff3BaseData

Things to think about before submitting:

• Make sure your changes compile and new tests pass locally.
• Add new tests or update existing ones:
  • A bug fix should include a test that previously would have failed and passes now.
  • New features should come with new tests that exercise and validate the new functionality.
• Extended the README / documentation, if necessary
• Check your code style.
• Write a clear commit title and message
  • The commit message should describe what changed and is targeted at htsjdk developers
  • Breaking changes should be mentioned in the commit message.

[lbergelson]

@lindenb This seems like a good idea but can we change the method name? getAttr and getAttribute are to confusing together. Something like getSingleAttribute() might be clearer?

[lbergelson]

I like this idea. Can we change the name though? Maybe getSingleAttribute()?

[lbergelson]

or getUniqueAttribute()

[lindenb]

@lbergelson thanks for the review ! I changed the name to getUniqueAttribute

[lbergelson]

👍

[lbergelson]

Thanks for all these additions @lindenb