ENH: add cupy tensor space

Author: Holger Kohr

odl/space/__init__.py

@@ -19,6 +19,9 @@
 from .npy_tensors import *
 __all__ += npy_tensors.__all__
 
+from .cupy_tensors import *
+__all__ += cupy_tensors.__all__
+
 from .pspace import *
 __all__ += pspace.__all__