Discuss DTF_FUN functors

[alexpearce]

In #144 we realised that we don't mention the DTF_FUN functor. This is a powerful way to use DecayTreeFitter, and is more explicit than TupleToolDecayTreeFitter. One can be more fine-grained with what's saved as well, which is nice.

We should mention it somewhere.

[vlisovsk]

@chrisburr @andriiusachov as you are teaching the DTF lesson this year, it might be worth updating the lessons with the DTF_FUN discussion.

[chrisburr]

I won't have time to before the lesson but I'll teach it if someone else can update it before I remind myself how it all works on Wednesday evening.

[andriiusachov]

I will run the lesson tomorrow, so I will be able to add example of DTF_FUN usage