I have a set of 'x' and 'y', and want to use a special function 'f(x)'
to fit it.
The function 'f(x)' contains three parameters.
But I'm confused by the three different procudure 'curvefit', 'lmfit'
and 'svdfit'.
It seems that all of them can meet my request, but I don't know the
difference between them.
