Improve Table Indexing

There is some limited support for table indexing such as:
```
new_table = document.tables[0][:5, :]
```
In order to select the first 5 rows of a given table.

However we need to:
- [ ] Adjust the returned Table such that its bounding box is tight with respect of the returned cells
- [ ] Support negative indexing

Start at the document.tables indexing entry point and inspect the current Table slicing behavior shown in the issue. Done means sliced tables have a bounding box tight to their returned cells and support negative indexing; add or update coverage for both behaviors.

Written by the indexing model from the issue text.