GH-4233: Add GeoSPARQL 1.1 centroid function

[edmondchuc]

GitHub issue resolved #4233

Adds the GeoSPARQL 1.1 geof:centroid function, also available through GeometryWrapper.centroid().

The function returns the planar, two-dimensional centroid of the input geometry as a Point. It uses JTS centroid semantics.

For GML inputs using a three-dimensional CRS, a non-empty XY centroid cannot be represented without inventing a Z ordinate, so the function raises an expression error.

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• Tests are included.
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See the Apache Jena "Contributing" guide.