function step(stepBy) {
            return function () {
                if (!_currentScene.currentImage) return;

                let nextId;
                if (stepBy === 1) nextId = _currentScene.nextImage.id;
                else nextId = _currentScene.prevImage.id;

                if (!nextId) return;

                const nextImage = _cache.images.forImageId[nextId];

                if (nextImage){
                    context.map().centerEase(nextImage.loc);
                    that.selectImage(context, nextImage.id);
                }
            };
        }
Loads the next image in the sequence
@param {number} stepBy '-1' if backwards or '1' if forward
@returns