Add sample data and obj2slpk example

[whatnick]

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) 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
• Blender
• Simplygon
• And any other implementations of the triangle-edge collapse and similar mesh simplification algorithms.
  Tile_+007_+008.zip

[whatnick]

Consider using fast_obj for easy OBJ parsing dependency with a permissive MIT licence.