Fatras: Bethe-Heitler calculation wrong?

While writing the documentation for Fatras, I've added some basic UnitTests to #1402 in order to create and show the principles of the particle interaction mode.

This is the test:
```c++
for (auto i : loop){
ActsFatras::Particle after = before;
const auto outgoing = process(gen, materialSlab, after);
fileOutput( before.energy()-after.energy() );
}
```

With `typeid(process) == BetheBloch` this gives a Landau distribution as expected:


While with `typeid(process) ==BetheHetiler` It looks to me as if the distribution shows what you would expect from `after.energy()/before.energy()`: