Split tables horizontally in split_elements if the column headers are big enough to make splitting hard

[HenryL27]

If column headers are too big, splitting a table doesn't really work because we attach the col header to all the subtables anyway. Instead, in this case, split the table in half horizontally instead of vertically - which splits the col headers too.

Breaks a table like this:

Note that the col headers are attached to all elements, despite the bboxes not reflecting that.

I'm really not sure that this is the ideal behavior