er.
  1) build a table of the frequency of each character on every line.
  2) build a table of frequencies of this frequency (meta-frequency?),
     e.g.  'x occurred 5 times in 10 rows, 6 times in 1000 rows,
     7 times in 2 rows'
  3) use the mode of the meta-frequency to determine the /expected/
     frequency for that character
  4) find out how often the character actually meets that goal
  5) the character that best meets its goal is the delimiter
For performance reasons, the data is evaluated in chunks, so it can
try and evaluate the smallest portion of the data possible, evaluating
additional chunks as necessary.
NrF