Validate keywords that are not allowed to be empty list using conlist

[StephanDeHoop]

In the EverestConfig we allow required keywords of type list to be empty. This is probably to easy testing, but for user doesn't really make sense. We can't run an optimization if we don't define any controls for which we are optimizing (unless I am missing something), same goes for objective_functions, if we don't specify an objective function what are we optimizing for?

TODO: Refactor this code to enforce required keywords of type list to be non-empty to improve the use of validating the config.yml (as required in actual use cases). This probably also means fixing associated tests that fail because we now require, for example, len(controls) > 0 (e.g., EverestConfig.with_defaults() will need to be changed too).

This relates to the config keywords: controls and objective_functions