[WIP] Attempt to make optimas generators compatible with generator standard

[RemiLehe]

Overall approach:

optimas generators currently call the ask and tell methods, which, under the hood, can the _ask and _tell methods.

In this PR:

• ask/tell are renamed to ask_trials/tell_trials and keep the same interface (for compatibility with the rest of the framework)
• _ask/_tell are renamed to ask/tell and adopt the new standardized interface.

This allows other packages to use the optimas generators. (In this case, these other packages will call the new, standardized ask/tell directly, without going through ask_trials/tell_trials.)

In order to wrap a standardized generator from another package, one would only need to make a thin wrapper around it that defines ask_trials and tell_trials.