Fix/duplicate namespace declarations

[pmcb55]

This PR is to fix an annoying problem with duplicate main-ontology namespaces appearing. For example, simply run against this Turtle to see the problem:

prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
prefix owl: <http://www.w3.org/2002/07/owl#>
prefix vann: <http://purl.org/vocab/vann/>

<https://ex.com/namespace-with-slash/> a owl:Ontology ;
     vann:preferredNamespacePrefix "ex" ;
     vann:preferredNamespaceURI <https://ex.com/namespace-with-slash/> .

<https://ex.com/namespace-with-slash/SomeClass> a rdfs:Class .

This results in Widoco generating: Table 1: Namespaces used in the document:

Prefix	Namespace
ex	https://ex.com/namespace-with-slash/
ex	https://ex.com/namespace-with-slash
owl	http://www.w3.org/2002/07/owl
rdf	http://www.w3.org/1999/02/22-rdf-syntax-ns
xml	http://www.w3.org/XML/1998/namespace
xsd	http://www.w3.org/2001/XMLSchema
rdfs	http://www.w3.org/2000/01/rdf-schema
vann	http://purl.org/vocab/vann

I would have added a simple regression test (using the Turtle I provided above), but I don't see any unit tests beyond the basic test/widoco/CreateDocInThreadTest.java file. There are very few tests in there, and I can't easily see how I'd assert on the correct behaviour, i.e., to assert that only the correct namespaces are listed in the generated output. Perhaps you could add such a test, at least as an initial example for others contributing to this codebase.

(This PR also introduces a blank line between the main ontology's namespace and the rest, just 'cos that's the most important namespace, and the blank line makes it stand out more - but feel free remove that if you prefer!)

[dgarijo]

Thanks @pmcb55 !