Adding sampling validation features

Related to issue #332 (closed)

In the current state, this MR simply produces the validation files containing features. The user can use them as it want outside the workflow.

Question:

• does this MR contains new steps to classify the validation files ?

Maybe the best solution is that each features requiring validation on these files must implement the related workflow:

• DL & GP to have a validation on the "test" dataset
• Regression to produce the validation of N models after learning and choose the best to produce the map