Set the KHIOPS_PROC_NUMBER env var for each training run using the value of max_cores

[tramora]

• the allocated number of CPU cores can never be greater than the value of max_cores

Fixes #585

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Implementation details :

• the changes to the runner added by @lucaurelien in PR Enable System Parameters Globally #622 are reported because they are needed in this one and will be useful when the API to set globally the system parameters is implemented

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TODO Before Asking for a Review

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[popescu-v]

See the comments.