…and retain expected *.ttl files content

`rdfpipe` utility is now used to regenerate *.rdf files what results in the
duplicated imports being discarded. Changes were made in Makefile file and
affects generation of *.rdf artefacts (owl-core, owl-restrictions and shacl
Makefile recipes) as well as conversion of these to TTL format
(convert-rdf-to-turtle). The latter were needed to keep the same TTL output
when using the new *.rdf files.

Scope of the changes:
* New generic convert-between-serialization-formats Makefile recipe
* The existing owl-core, owl-restrictions and shacl Makefile recipes now include an RDF regeneration step
* The existing convert-rdf-to-turtle Makefile recipe now uses the same namespaces that are used for XSLT transformation (test/ePO-default-config/namespaces.xml) to provide rdfpipe with prefixes so the output makes use of compact URIs.
* As the reported issue occurs for EPO v4.2.0 which introduces some new prefixes, these prefixes have also been added to test/ePO-default-config/namespaces.xml so it would be possible to reproduce the fix.

…akefile

…to depend on the new file

The new XML contains user namespaces (defined in namespaces.xml) and internal
namespaces (such as core-shape). Both model2owl internals and get_namespaces.sh
relies on that file and not on namespaces.xml. Moreover, model2owl internals now
depends on a new template which generates namespaces declarations based on
enriched file which is passed to the XSLT transformation engine as command line
argument. For now, the path of the new XML file is kept in the Makefile, but
this can be further improved in the future (if we come up with application
internal XSL config or other solution).

Scope of changes:
* new XSL file for generation of the enriched namespaces XML file
* unit test for the above XSL file
* project codebase:
  * new function generating namespaces declarations and references to
    `$namespacePrefixes` replaced with `$internalNamespacePrefixes`
* Makefile changes:
  * new recipe
  * existing 3 recipes generating *.rdf artefacts now include generation of the
    new XML file
  * required adjustments
* Adjustments in the get_namespaces.sh script

… pass new namespaces file as a parameter for the rest of xslt transformation recipes in the project

Document use of the new `NAMESPACES_USER_XML_FILE_PATH` variable.
Any recipe without a docstring (comment documenting its use) that
uses the new variable now got a description.