cuda.core.system: add conveniences to convert between device types

[mdboom]

This adds two convenience methods to convert between cuda.core.Device and cuda.core.system.Device.

I have a few design questions for this one:

• Is the naming of Device.to_system_device and Device.to_cuda_device ok?
• This could be a new type accepted by the constructor, e.g. cuda.core.Device(nvml_device), but that requires importing the other module from the constructor which could be a performance / cyclical import nightmare.