getExpandLevels() {
			const aExpandInfos = Object.values(this.mPredicate2ExpandInfo);
			return aExpandInfos.length
				? JSON.stringify(aExpandInfos.map((oExpandInfo) => {
						// build the server representation
						return {NodeID : oExpandInfo.nodeId, Levels : oExpandInfo.levels};
					}))
				: undefined;
		}
Returns the "ExpandLevels" parameter to the "TopLevels" function describing the tree
state in "$apply".

@returns {string|undefined}
  The "ExpandLevels" parameter or undefined if no tree state is kept

@public