Watch Cuda context creation at imports

When importing some modules (ex. scikit-cuda), a Cuda context might be created.

However, when using Threads and especially Processes, a prior context creation is not compatible with a new context creation (see for example here).

As nabu might be used in a Thread/Process (tomwer), it is crucial to avoid creating contexts before it is actually used (ex. "cuda processing class" instantiated).

This problem is even worse when the nvidia-persistenced daemon is running (DGX-1, power9).