[Rivet] HepMC Python module

[Frank Siegert]

We had the discussion of what to do with the HepMC python module, which we 
use in both Rivet and AGILe at the moment. The possible solutions we came 
up with were (please amend if necessary):

1. Have it added to HepMC itself.
   + All packages could use it w/o additional dependencies
   - It will probably take forever and a year, and even once it is in,
     it will take a year until updates/fixes will be made.
   ==> not a real option.

2. Have it as a standalone package.
   + Clean solution, available for all packages.
   - Package maintainance for one simple Python module.
   - Rivet/AGILe will depend on yet another external package.

3. Have it in Rivet and AGILe.
   + Easy and pragmatic.
   - Problems when multiple of these are installed into a common prefix
     (think un-installation or even .deb packages for our PPA)
   - Other packages which might want to use the module have to depend on
     either Rivet or AGILe.

I would like to propose a fourth option:

4. Move AGILe's HepMC output functionality (it's only a few lines) into 
   the C++ part, such that we don't need the python HepMC module there. 
   This way we simply distribute the module with Rivet, until some better
   solution (maybe even distribution within HepMC) becomes feasible.
   + Easy and pragmatic.
   - Other packages which might want to use the module have to depend on
     Rivet.

Actually thinking about it now, I don't even see that we couldn't do 
something similar for Rivet, such that we don't depend on the HepMC python 
module at all. But I might be stirring Andy's wrath here ...

Frank