Improve verbose mode

[hakkelt]

• summary function added to the IterativeAlgorithm struct. This function returns a tuple of pairs where the key is the column title. E.g.: ("" => it, , "f(xg)" => state.f_xg, ...)
• The display function is modified to call summary and display the result. When it = 0 is passed, then only a table header is printed.
• When freq in IterativeAlgorithm is set to 0, then only a single line is printed after the iteration stops. The format of this line is like: "total iterations = 43, f(xg) = 3.524e-3, ..."
• default_display function now accepts printfunc optional argument. The default value is println, and this argument allows replacing it, e.g., with a proper logger.