[test][centroid] Add test for big doubles.
[test][to_svg] Add geom_to_svg() helper function.
[test][centroid] Improve the test for Polygon using big coordinates.
[centroid] Reduce numerical errors by translating the Geometry closer to 0 and then the result back.

The first Point of the Geometry is used as the new origin.
During the centroid calculation each Point is translated by subtracting the origin.
At the end the resulting point is translated back by adding the origin.
[centroid] Disable error-reducing translation for other coordinate systems than cartesian.

Remove undefined namespace.
Add missing headers.
Add a note about other coordinate systems.
Add copyright info.
[centroid] Disable translation for non-areal Geometries.
[centroid] Move translating_transformer to separate file.

Add ctor taking Geometry to transformer.
This allows to encapsulate the origin-retrieving-logic inside the transformer.
Remove assign_origin() function.
Merge pull request #155 from awulkiew/fix/centroid

Fix/centroid