Not only better readability...

* Removed operator<< and operator>> from SasAnalysis*, replaced by
  write and read methods
* SasAnalysis*::read and SasAnalysis*::write handle vectors instead of
  raw arrays allocated on heap, even if the continue to use them
  internally. To do that a std::copy is used instead of the direct
  access, leading to a little cpu and memory overhead
* Even better readability for Pittpi::fillgroups