[Halld-offline] hdgeant modifications

[Beni Zihlmann]

Hi Kei,

I committed a few changes to hdgeant and bggen. Now I hope the relation 
between parent
and daugther particles is more or less restored. The initial thrown 
particles have an myid
number which is the track number of the particle and parentid is zero. 
All intermittent
particles from bggen have myid number zero and parentid zero. If 
parentid is not zero
than that number is the track id number of the parent which is myid. 
However if you
can not find a parent particle with that myid that means the parent 
particle was
generated inside the calorimeters. You will find that rather often you 
see two photons
being associated with a parent particle that does not show up in the 
list. That means
the parent pizero was generated inside the calo or just at the edge but 
the decay
vertex is outside. I think this is an edge effect were in gustep the 
particles being saved
into an new vertex is slightly different than particles being made 
primary. I will check
on that. The upshut is at the moment that you will find some particles 
that you can
not associate with a parent.

cheers,
Beni