This introduces:
* asyncronous event generation
* possibility for parallel event generation

This is useful for:
* hiding latency (IO) of certain generators
* decoupling the actual work from the call sequence into HybridGenerator
* collaboration from multiple clones of the same generator to generate
  a certain number of events

The implementation relies on tbb::task_arena and input/output queues
for decoupling the task_arena from the HybridGenerator thread.

An example is added for parallel event gen.

In addition, this commit has the following changes:

- Small adjustments to seeding of Pythia8 in order
  to avoid same seeds in multiple parallel Pythia instances.

- possibility to init external generator from an INI file (typically
  done or available in O2DPG)

- use shared_ptr instead of unique_ptr to keep generators in order
  to avoid lifetime problems with async processing

- preparion to run underlying generator Init() functions in async way
  (not active yet; needs checks if generators are thread safe)