add conveniences

This is a long-overdue convenience function:

```c++
giskard_core::QPController generate_controller(const std::string& yaml_string)
{
  YAML::Node node = YAML::Load(yaml_string_);
  giskard_core::QPControllerSpec spec = node.as<giskard_core::QPControllerSpec>();
  giskard_core::QPController controller = giskard_core::generate(spec);

  return controller;
}
```

Please, add it.