Provide generic Index for all columns

[mbastian]

In Gephi's Appearance module we have a lot of conditional branches on whether the column is indexed or not. We have logic to get values and counts for dynamic, non-indexed columns. It would be simpler if GraphStore could provide the functionalities of the Index even for non-index columns. In other words, I think we should always provide an Index for each column. The underlying Index implementation could fallback to element iteration.