Construct TableBundle for an iterable of Table objects

Currently `TableBundle`'s constructor requires a `BlockIterator = Iterable[Tuple[BlockType, Optional[Any]]]`. 
Could be nice to also allow creating a `TableBundle` from a plain `Iterator[Table]`. 

Currently this requires some bending backwards:

```
tables = [table1, table2, ...]  # each of these is a Table object
b = TableBundle((BlockType.TABLE, t) for t in tables)  # bit annoying!
```