Convert float numbers (double) to fractions (string)

Convert signed float numbers to signed fractions as strings. Matrix-compatible.

R = double2fraction(X,tol) returns a cell array R containing strings of signed fractions for double numbers stored in N-dimensional matrix X and within an optional tolerance tol (default 0.01).
Does basically the same job as R = rats(X), but rats takes the string length as a restricting condition, while the provided function takes the absolute error tolerance.
See example.m for usage.