Remove side-effects when running several proceses of n-fold cross-validation of the same model while sharing the same grobid-home directory

[lfoppiano]

This PR allows to run the n-fold cross-validation of a specific model several times sharing the same grobid-home.

Currently the models are named model_name_nfold_x with model_name corresponding the name of the model and x the n-ith fold. However if the same model is evaluated at the same time, they will write/overwrite the models in the grobid-home/tmp directory.

This PR:

• add a unique random string on the ith-fold model name
• collect generated files paths and deleted them at the end of the process

[coveralls]

Coverage decreased (-0.01%) to 38.001% when pulling 67bf75a on feature/randomise-model-name-during-nfold into 47c8978 on master.

[kermitt2]

Thanks Luca, tested and it works fine.