Increase max_adjacent_empty to prevent form breakage

[yanokwa]

Sometimes, when I'm iterating on forms, I like to use a lot of blank rows to mark my place. And because of #612, my form doesn't get converted by pyxforms.

max_adjacent_empty of 20 is fine for columns, but for rows, I think we should do 100.

If it's easy, maybe we should also warn that we stopped processing the form because of lots of empty rows and columns.