vtkGLTFReader API changes and fixes

This Merge request attempts to fix issues that arose when using the vtkGLTFReader (implementing a mapper & paraview plugin).

It fixes a few bugs and adds new methods to the reader:

• Rename some parameters and change some types (size_t to vtkTypeId).
• Add GetAllScenes method to get an array of all scene names. Add SetScene method to set a scene using its name instead of index.
• Use vtkDataArraySelection to enable/disable animations.
• Fix a bug that would cause gltf textures to not be exported properly.
• Export the default material values even when no texture image is present.
• Use the default material when no material is present.
• Fix a rounding issue when generating time steps.
• Link number of time_steps to currently enabled animations (previously was all animations).
• Fix bugs with the output not being updated correctly when some parameters were changed.
• Fix bug where animated objects would not be restored to their initial state when animations got disabled.
• Update animation tests to reflect the change in time step generation.
• Change time steps from integer indices to time values, fix tests accordingly.