Implements #68 gpu selection

Not tested with second GPU but should work.

applications/reconstruct/include/ftl/voxel_scene.hpp

@@ -78,6 +78,7 @@ class SceneRep : public ftl::Configurable {
 	void debugHash();
 
 	cudaStream_t getIntegrationStream() const { return integ_stream_; }
+	int getCUDADevice() const { return cuda_device_; }
 
 	private: