Remove the gather_index in DataAssembly

Hi, I suggest removing `gather_index` in any `DataAssembly` types due to the following reasons:

1. it always create bugs when one dimension only has one coordinate. This creates big confusion for new users;
2. it has no practical use;
3. occasionally, multi-index creates more trouble than the normal index: e.g., when we need to add more coordinates.

Though this issue is in brainio repo, I am raising it here since it is more relevant in context.