_addSourceToParent(source, sourceType) {
        // add the source if it is not already in the correct parent assets array
        if(sourceType === "image") {
            const parentAssetArray = this._parent["images"];
            const isInParent = parentAssetArray.find((element) => element.src === source.src);
            !isInParent && parentAssetArray.push(source);
        }
        else if(sourceType === "video") {
            const parentAssetArray = this._parent["videos"];
            const isInParent = parentAssetArray.find((element) => element.src === source.src);
            !isInParent && parentAssetArray.push(source);
        }
        else if(sourceType === "canvas") {
            const parentAssetArray = this._parent["canvases"];
            const isInParent = parentAssetArray.find((element) => element.isSameNode(source));
            !isInParent && parentAssetArray.push(source);
        }
    }
Adds the source to the correct parent assets array

     params:
     @source (html element): html image, video or canvas element that has been loaded
     @sourceType (string): either "image", "video" or "canvas"
*