implement scaffold and two simple examples

[bertiqwerty]

Motivation

Scaffold for convergence criteria. A strategy has converged if it is not expected provide better results in future requests. Even though it might overstate the math. term convergence I picked it over stopping criterion or termination criterion, because the latter can also just the experimental budget has been reached. But these convergence criteria are part of the strategy and have access to the internal state to assess the progress of the optimization. The orchestration layer, on the other hand, with, e.g., the number-of-experiments-condition or now the new has-converged-condition can stay in the step-wise strategy.

Have you read the Contributing Guidelines on pull requests?

Y

Have you updated CHANGELOG.md?

y

Test Plan

unit tests