Until now, fitting was available as part of the Statistics program. The
overhead of being part of Statistics was a burden on the development of
this feature and in particular in adding new functionality: 2D fitting in
particular.

With this commit, the fitting features of Statistics have moved (and
expanded) into a new Fit program with the executable name 'astfit'. The
small tutorial within the Statistics program has also been moved into the
Tutorials chapter and also expanded to describe the new functionality.