Cuda devices monitoring

[igor-davidyuk]

An attempt to allow assigning GPU devices through OpenFL.
The PR introduces an optional 'device monitor' plugin for Envoy and two information flows:

1. GPU status goes from Envoy through Director to the Frontend.
2. GPU utilization policy that goes the same path but in reversed order.

The Director_Pytorch_Kvasir_UNET example is modified to utilize the new infrastructure.
There are 2 envoys, one that utilizes GPUs and one that does not.

Device assignment for an experiment is done through 'device assignment policy' which may be ' CUDA_PREFFERED' or 'CPU_ONLY'

TODO:

• Update docs
• Update example Envoy config files
• Update interactive API tests

Code discussion was in #162