Improve turtle style

[cthoyt]

Nico asked me to double check the content in #88 (comment) and I realized that the turtle was very difficult to read. This PR updates the way the turtle is output (but not its content).

It mostly rearranges indentation and where semicolons ; are put. A notable change is I use nested objects for dbxrefs, which reduces some more visual clutter such as in

NCBITaxon:121616 a owl:Class ;
    rdfs:label "Micromonospora matsumotoense"^^xsd:string ;
    rdfs:subClassOf NCBITaxon:1873 ;
    ncbitaxon:has_rank NCBITaxon:species ;
    oboInOwl:hasDbXref "PMID:8897428"^^xsd:string ,
        "PMID:8897428"^^xsd:string ,
        "PMID:10499285"^^xsd:string ,
        "GC_ID:11"^^xsd:string ;
    oboInOwl:hasOBONamespace "ncbi_taxonomy"^^xsd:string .

I checked with python3 src/ncbitaxon.py build/taxdmp.zip ncbitaxon.ttl and robot convert -i ncbitaxon.ttl -o ncbitaxon.owl that this still makes valid and correct turtle that can be converted to OWL

[jamesaoverton]

The new output looks nicer, but it's an exaggeration to say the old output is "very difficult to read".

It's good that this PR doesn't break the syntax (your robot convert check). It's also important to check that it doesn't change the content.

[anitacaron]

@cthoyt Can you fix the conflict, please?