Morph-kgc

Support of [Natural mapping of SQL values](https://www.w3.org/TR/r2rml/#natural-mapping)

- 67: [Natural mapping of SQL values](https://www.w3.org/TR/r2rml/#natural-mapping)
- 67: Integers generated without .0

Solving 62.

- Support for attributes in XPath.
- Support for remote JSON and XML files.
- Support for remote mapping files.

Minor performance enhancements.

This major release features **[RML-star](https://kg-construct.github.io/rml-star-spec/)**.
- Support for [RML-star](https://kg-construct.github.io/rml-star-spec/).
- Integration with [Oxigraph](https://oxigraph.org/pyoxigraph/stable/index.html) via `materialize_oxigraph`.
- _XPath 2.0_ has been replaced by _XPath 3.0_.
- `clean_output_dir` has been removed as a parameter in the config file. Now Morph-KGC truncates only RDF output files by default.
- It is now possible to generate multiple (smaller) output RDF files using `output_dir`.
- RML-star test cases added to continuous integration.
- New documentation replacing the Wiki.

- Datatype inference for _SQLite_.
- Handling curly braces in string templates according to [R2RML spec](https://www.w3.org/TR/r2rml/#dfn-template-valued-term-map).
- Do not generate output dir when executing as a library.
- Fixed bug regarding the generation of integers values as float for _JSON_ files with _NULL_ values.