Various fixes for StateAttributeCacheHelper: fix the use of a custom limits cache dictionary (it was being accidentally ignored before because cache or {} always returned a new dictionary if cache={}; make it possible to update the cache on-the-fly (useful when restoring states); don't update cache if attribute is None, and minor fix to docstring.
Various fixes for StateAttributeCacheHelper: fix the use of a custom limits cache dictionary (it was being accidentally ignored before because cache or {} always returned a new dictionary if cache={}; make it possible to update the cache on-the-fly (useful when restoring states); don't update cache if attribute is None, and minor fix to docstring.

stscieisenhamer/glue,stscieisenhamer/glue