Prior to this commit CudaDataManager::GetGPUBufferPointer() returned a
pointer to the GPU buffer pointer. Its return type should have been void**.
Every usage of this method across RTK and CudaCommon dereference the
returned pointer pointer, so just return the buffer pointer directly as it
required for future commits adding support for __cuda_array_interface__

Adapt ITK's PyBuffer approach to inject __cuda_array_interface__ in wrapped
code.
Closes RTKConsortium#10

This allows modules like RTK to easily reuse files from the CudaCommon
sources, e.g. CudaImage.i.init and CudaImage.i.in