Gets the logical layout of the table as a rectangular grid of cells. It considers rowspan and colspan. A cell with a large span is represented in the grid by being referenced several times. The tablePortition parameter can get just a <thead>, <tbody>, or <tfoot> portion if you pass one.

Note: the rectangular grid might include null cells.

This is kinda expensive so you should call once when you want the grid, then do lookups on the returned array.