Change `clang-format` style to have line breaks in c'tor initializer

We use the `clang-format` style of ROOT for all the new source files in
RooFit, but some code looks pretty ugly. For example, here in the
constructor of `RooNLLVarNer`:

```C++
RooNLLVarNew::RooNLLVarNew(const char *name, const char *title, RooAbsPdf &pdf, RooArgSet const &observables,
                           bool isExtended, RooFit::OffsetMode offsetMode, bool binnedL)
   : RooAbsReal(name, title), _pdf{"pdf", "pdf", this, pdf}, _observables{getObs(pdf, observables)},
     _isExtended{isExtended}, _binnedL{binnedL}, _weightVar{"weightVar", "weightVar", this, *dummyVar(weightVarName),
                                                            true,        false,       true}, ...
```

Really not great to have all the elements in the constructor initializer
list in one line. It's hard to read, there are weird line breaks, and if
you only change one member it looks like you have changed everything.

This commit suggests to have line breaks in the style instead, so from
now on, it will look better.