zpk function "corrects" round numbers to nearby values.
Using python bindings, Jeff Kissel found that creating a filter with zpk(10.0, 0.4, 1.0)
led to values that were off by one or two bits in the double precision mantissa.
He later manually changed the foton file back to the round values. Foton loaded the file without generating any warnings or "correcting" the function arguments.