feat: Cross-validation reporter with accessors

[MarieS-WiMLDS]

Is your feature request related to a problem? Please describe.

Current way to access cross_validate is a bit weird for users. It does things on their behalf that they do not really expect (in particular: putting). Also, the value related to this feature exists, but it too scarce to explain a change, and the form of the interaction with the user prevents us from doing more.

Describe the solution you'd like

Adding a CrossValidationReporter objects allows to answer the needs and problems listed above, while respecting our needs to keep the change simple for the user.

Describe alternatives you've considered, if relevant

No response

Pre-requisites

• [ ] Complete #807
• Full description with validated functional requirements
  • [ ] How does the CrossValidationReporter appear in activity feed? (if different from a view)
  • [ ] #803
• Technical specifications
  • [ ] Python: Function signatures and interfaces defined

[MarieS-WiMLDS]

Cleaning necessary

• remove matplotlib option
• remove former cross_validate function
• other ?