Pythia8 seeding improvements

[sawenzel]

This commit improves the seeding of Pythia8 in O2. The seeding...

(a) ... is now done as part of the object's Init function automatically. Users are no longer required to provide own seeding logic, which can significantly simplify the setup. By default, Pythia8 will seed against ROOT TRandom::GetSeed, which is itself set to values of the command line option --seed, used in the o2-sim ... or o2-sim-dpl-eventgen execetuables (which are the 2 places undertaking event generation). This setup guarantees that

```
o2-sim-dpl-eventgen --generator pythiapp --seed x
```
will result in different event sequences when x changes.

(b) Users can simply set the seed via a setInitialSeed function on the GeneratorPythia8 object. The function must be called before GeneratorPythia8::Init is executed. So calling it right after the constructor is fine.

Example code (e.g., inside user Generator macro) is:

```
auto mygen = new o2::eventgen::GeneratorPythia8();
long seed = atol(getenv(ALIEN_PROC_ID));
if(!mygen->setInitialSeed(seed)) {
  std::cerr << "seeding failed";
}
```

In result, the commit leads to a simplification of the Pythia8 setup also in GeneratorFactory.