Value error raised in xlw_opblocks.py when there are more 3276 rows of data in a sheet.
That axception is raised by chr() function (line 212 of module xlw_opblocks.py) applied to 'op' list that contains the element O1 > 255.
O1 is greather than 255 if 'offset' variable is greather than 65535 in line 210 of same module.
offset is greather then 65535 when there are more than 3276 rows in...
