Extract simple table could be more efficient

[jstockwin]

At the moment we're looping through the reference row and then the reference column, and computing the element which is in line with both the reference row element and the reference col element.

This means we're doing to geometry checks to work out each row len(cols) times, and the checks to work out each col len(rows) times.

We should do the geometry filtering at the start to create a list of rows and a list of cols. We can then just & the two to get the element.

This should require much less processing as you only do geometry checks len(rows) + len(cols) times, rather then len(rows) * len(cols) times.