Add sample data and obj2slpk example

I am working on an example using this library to convert SFM derived urban models to SLPK format. I have the attached model tile to donate under creative commons ([CC-BY-SA](https://creativecommons.org/licenses/by-sa/2.0/au/)) licence to this library and use as a focus for development for this functionality.

I can supply the model as a PR as well once we discuss appropriate folder structure under which testdata should sit in the repository.

The obj2slpk function proposed will follow the raster2slpk pattern:

`obj2slpk input.obj input_lod1.obj input_lod2.obj ... output.slpk`

To avoid extra dependencies being added to this repository it is recommended that the LOD's for the given OBJ are generated separately using tools such as.

- [VCGLib](https://github.com/cnr-isti-vclab/vcglib/blob/master/apps/tridecimator/tridecimator.cpp)
- [Blender](https://docs.blender.org/api/current/bpy.ops.mesh.html#bpy.ops.mesh.decimate)
- [Simplygon](https://documentation.simplygon.com/SimplygonSDK_8.3.36300.0/articles/simplygonui/Reduction.html)
- And any other implementations of the [triangle-edge collapse](https://dl.acm.org/doi/10.1145/258734.258849) and similar mesh simplification algorithms.
[Tile_+007_+008.zip](https://github.com/Esri/i3s-lib/files/6088294/Tile_%2B007_%2B008.zip)