Enable attributes in vtk wrapper functions

[benjaminum]

This PR

• enables attributes in vtk wrapper functions allowing to preserve (interpolate) vertex and triangle attributes
• fixes a segfault when converting empty vtkPolyData to a TriangleMesh

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This change is 

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[ssheorey]

Reviewable status: 0 of 5 files reviewed, 2 unresolved discussions (waiting on @benjaminum and @yxlao)

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cpp/open3d/t/geometry/VtkUtils.cpp line 306 at r2 (raw file):

/// \param copy If true always create a copy for attribute arrays.
/// \param include A set of keys to select which attributes should be added.
///                The special key "*" includes all attributes.

"*" may hint that globbing / regexp is supported. Perhaps use a special constant (e.g.: TensorMap::ALL_KEYS) instead?
Also. document precedence of include and exclude. From code below, looks like exclude has precedence over include. i.e. if include = {"colors"} and exclude = {"colors'} then "colors" is excluded. Or should this be an error?

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cpp/open3d/t/geometry/VtkUtils.cpp line 340 at r2 (raw file):

                    "Ignoring attribute '{}' for TensorMap with primary key "
                    "'{}'",
                    key_tensor.first, tmap.GetPrimaryKey());

We shouldn't need a LogWarning for doing what the user asked us to do, i.e. if they explicitly asked to exclude "colors", no need to warn them about this.

[benjaminum]

@yxlao I added another getter without const to simplify some conversion code. Is there a reason for just having the const version? Same for TriangleAttr and in PointCloud.h

[benjaminum]

Reviewable status: 0 of 7 files reviewed, 3 unresolved discussions (waiting on @ssheorey and @yxlao)

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cpp/open3d/t/geometry/VtkUtils.cpp line 306 at r2 (raw file):

Previously, ssheorey (Sameer Sheorey) wrote…

"*" may hint that globbing / regexp is supported. Perhaps use a special constant (e.g.: TensorMap::ALL_KEYS) instead?
Also. document precedence of include and exclude. From code below, looks like exclude has precedence over include. i.e. if include = {"colors"} and exclude = {"colors'} then "colors" is excluded. Or should this be an error?

Done. I changed to signature to avoid the special "*" element and improved the docstring.

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cpp/open3d/t/geometry/VtkUtils.cpp line 340 at r2 (raw file):

Previously, ssheorey (Sameer Sheorey) wrote…

We shouldn't need a LogWarning for doing what the user asked us to do, i.e. if they explicitly asked to exclude "colors", no need to warn them about this.

The function is not really meant to be directly used by the user and we decide which attributes to keep/exclude based on the algorithm we want to use.
However, if a user wants to combine o3d and vtk in C++ they can use the function; we don't hide the functionality.