Add __bool__, __iter__ and __len__ to TableFinder

[felipdsa21]

The TableFinder class (returned by Page.find_tables()) is extended so it can be used like the list of tables it holds:

• len(page.find_tables()) returns the number of tables found. Before, it raised TypeError.
• bool(page.find_tables()) is False when no tables were found. Before, it was always True, so if page.find_tables(): never skipped pages without tables.
• for tab in page.find_tables(): iterates over .tables.

.tables and indexing work as before.

Adds test_table_finder_len_bool_iter() to tests/test_tables.py. It checks all three methods on a page with three tables (test_3179.pdf) and on a page with none (battery-file-22.pdf).

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[felipdsa21]

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