Support nesting of AlignedDynamicTable

[oruebel]

As identified #571 and #574 while nesting of AlignedDynamicTableis in principle possible---i.e.,we could use an AlignedDynamicTable as a category for another AlignedDynamicTable--- this behavior is currently not supported. In particular the interface functions of AlignedDynamicTable would need to be updated to support this, e.g.:

• AlignedDynamicTable.__getitem__ does not support more than 3 elements in a selection tuple and would need to be updated to support nested selection behavior.
• AlignedDynamicTable.to_dataframe will result in error because MulitIndex on the columns would be created with varying numbers of levels for different parts. This would need to be fixed.
• To prevent the user from nesting AlignedDynamicTables, Add aligned table tutorial #571 added error checks in `AlignedDynamicTable.initandAlignedDynamicTable.add_category`` which would need to be removed if supported.

Checklist

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[oruebel]

There is no explicit need for this feature right now, but could be useful to have. I'm closing this issue for now until we have a real use case. I created this issue mainly for documentation purposes and in case the need should arise in the future.