added a -gpu flag so that the device could be specified
added a new attribute to the scene.xml <scene_paths> tag called 'is_data_rel'

the scene.xml file can now look like this
<scene>
  <scene_paths path="./" is_data_rel="true"></scene_paths>
</scene>

If is_data_rel is set to "true" then the (data) path attribute shall be made relative to the scene.xml file.

That way, if we call boxm2_adaptor::load_scene("model/scene.xml") and we want the model files to be stored in the 'model' directory, then we can just set <scene_paths path="./" is_data_rel="true">, instead of <scene_paths path="./model/">. is_data_rel is false by default, which preserves the original behavior.
better error handling in boxm2_load_scene_process.cxx

boxm2_load_scene_process now returns failure when it cannot parse the scene.xml file.
refactored attribute name