LookupSource in which consists the list of keys composed of the value of number specified by levels and the list of values corresponding to these keys.
It is implemented in tree structure internally for the performance of looking up the value by key. This class object is not created directly, but will be initialized as GridBase.lookupProvider or added to GridBase.addLookupSource through setting object. It specifies "type" as "tree" in setting object. Even if do not specify "type", it will also be considered as LookupTree.