hypertreegrid data is in a pointdata collection, it would be useful to put it elsewhere
The pointdata arrays must all have the same number of tuples are there are points in the dataset. We don't really have points in HTG and it would be useful if different arrays could have different lengths. For example all empty cells could point to the same entry.
We should move the arrays to the fielddata, or make up a new collection type, analogous to table and graph's specialized array collections.