[WIP] Added trial count to delete_trials message

[mgermain]

Minimal edit to fix #653, I also tried to stick to the recommendation in the issue.

There are much better ways to do this. but that would require a deeper dive into the code.

What do you suggest?

[bouthilx]

Good enough! We can optimize this latter on if needed. Could you please do it as well for delete_experiments and add a test for each? Tests would be here. You can simply verify if the correct number of trials is printed using capsys (doc here).