working on the tutorial

splitting a print() function in GammaComboEngine to have just
a list of availabe PDFs
improving the 2D chi2 plots
changing what happens if chi2 too small

previously, we set chi2=0 if the observed value was less than 1e-10.
Now only when we find chi2<0. The motivation for this change is that
we got warnings when running the tutorial with the circle.
changing the config system for the tutorial

we moved from the enum and switch/case based system to one
that only uses strings. This seems a lot easier, and in particular,
it saves the user from having to mess with yet another file where
stuff needs to be configured.

Also, the enum based system is very tricky to break down to a
modular level- so far, all enums had to be globally defined for
all combination projects...