Add extraction functions for tune_*vlmc results

[fabrice-rossi]

Currently the only simple use of tune_*vlmc results consists in converting them to *vlmc objects. It would be interesting to add other uses such as:

• plot functions
• retune functions: this could be used to get directly the best AIC model from the BIC one and vice versa without running the full search again
• possibly a direct extraction of the best AIC model or BIC model when all models where saved

Ongoing work:

• plot functions
• autoplot functions for more complex representations
• retune functions