[Feature Request] Table structure extraction at the API

There is already some table detection mechanism in tesseract but unfortunately, there is seems to be no possibility to access the table structure at the API.

This could be done only minimal changes to the API, just by expanding the PageIteratorLevel enum by two additional members RIL_TABLEROW and RIL_TABLECELL or similar. Those would only be relevant inside PT_TABLE blocks, just like PT_PARAGRAPH is only meaningful for text blocks.