Fit results save and load functionality

[QuanticPony]

After the optimization is complete the user should be able to save it. And maybe load it again if wanted.

Proposed behavior
Something like this:

fitter = itfit.Fitter(xdata, ydata)
fitter()
plt.show()

fitter.get_last_fit().save_fit("example.txt")

We probably want 2 files with:

• Data selected.
• Optimization details. Statistics and fit related things, as function used.