Removes `iterator` from `Synfit`

I removed the `iterator` function from the `Synfit` class because I need
it in two separate parts of the class. And also because it seems to be a
general purpose function that I could use somewhere else.
Creates a fitting package

This package contains a routine to automatically fit the spectral lines.
The routine was strongly influenced by Ivan Hubeny's `synfit`. In short,
it synthesize spectra in a set of values and compare with the
observation by calculating the chi-square. The minimum chi-square will
give the best values.

There is still room to improve, like applying a convergence method.