Behaviour of shift

[scraperdragon]

Shift checks whether the shifted cell exists with assert_one.

Not convinced it should -- it might not exist at the sides of a sheet etc.

  File "/home/.local/lib/python2.7/site-packages/xypath/xypath.py", line 626, in shift
    t_cell = self.table.get_at(b_cell.x + x, b_cell.y + y).assert_one()