Correct calculation in ProjectPointToPlane (backport #702)

[mergify]

🦟 Bug fix

Summary

Use unit normal for projection in gz::math::ProjectPointToPlane.

Before this change the function used the unnormalised normal to calculated the projection of the interpolation point onto one of the grid parallelpiped surfaces. This caused the gz::math::TrilinearInterpolate to return an incorrect interpolated value. These functions are used in the TimeVaryingVolumetricGrid, which in turn is used by the Environment system. The upshot is that this system incorrectly interpolates data whenever the trilinear interpolant is used. It is apparent when examining volumetric data using the PointCloud GUI widget, as the ranges displayed in the widget do not correspond to the underlying data provided to the Environment preload system.

After this change the PointCloud display is consistent with the input data.

The unit test InterpolationPoint_TEST does not catch this case, because the data is provided for a unit cube, and the normals calculated for this cube all have unit length, so the effect of scaling the normal is not tested. This is also the case for the environmental system tests in gz-sim. It may be better to use non-unit grids for testing systems that involve geometric calculations involving vector operations that depend on normalisation.

There is a work-in-progress example here (https://github.com/srmainwaring/gz-math/tree/prs/volumetric-grid-example) that highlights the issue for a 4 x 4 x 4 grid sampled from a wind field.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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This is an automatic backport of pull request #702 done by Mergify.