Creator should be able to provide a single solution

[julian-zucker]

Is your feature request related to a problem? Please describe.
We consistently have to make creators with this shape:

Vector.fill(32)(... the actual creation part...)

because of the type signature of CreatorFunction.create(), namely () => TraversableOnce[Sol]'

Describe the solution you'd like
We should keep the current creator signature, for people who want some kind of more complex construction method, that yields more than one solution. We should add something like a "SingleSolutionCreator", similar to the "MutatorAgent"/"ModifierAgent".

Describe alternatives you've considered
This is low priority, as it really only saves people typing Vector.fill(32)(…). But I think this type signature will be confusing, so it would be nice to provide a more intuitive/less surprising signature for this method.

Additional context
Add any other context or screenshots about the feature request here.