Resurrect beams

[Juergen Reuter]

When not specifying beams, the program gives mismatches in the density matrices:

 Evaluator product
 First interaction
 Interaction: 3
 Outgoing:
 Particle 1
 E =   5.000000000000E+02
 P =   0.000000000000E+00  0.000000000000E+00  5.000000000000E+02
 mask [fch] = [FFgT]
 source: (2)1
 Particle 2
 E =   5.000000000000E+02
 P =   0.000000000000E+00  0.000000000000E+00 -5.000000000000E+02
 mask [fch] = [FFgT]
 source: (2)2
 State matrix:  norm =  1.000000000000E+00
 []
   [] => ME(1) = ( 1.000000000000E+00, 0.000000000000E+00)

 Second interaction
 Interaction: 5
 Incoming:
 Particle 1
  [momentum undefined]
 mask [fch] = [FTT]
 internal links: X => 3 4
 source: (4)1
 Particle 2
  [momentum undefined]
 mask [fch] = [FTT]
 internal links: X => 3 4
 source: (4)2
 Outgoing:
 Particle 3
  [momentum undefined]
 mask [fch] = [TTT]
 internal links: 1 2 => X
 source: (4)3
 Particle 4
  [momentum undefined]
 mask [fch] = [TTT]
 internal links: 1 2 => X
 source: (4)4
 State matrix:  norm =  1.000000000000E+00
 [f(2)]
   [f(-2)]
     []
       [] => ME(1) = ( 0.000000000000E+00, 0.000000000000E+00)
 [f(1)]
   [f(-1)]
     []
       [] => ME(2) = ( 0.000000000000E+00, 0.000000000000E+00)
 [f(3)]
   [f(-3)]
     []
       [] => ME(3) = ( 0.000000000000E+00, 0.000000000000E+00)

******************************************************************************
******************************************************************************
*** FATAL ERROR: Product of density matrices is empty
***                 --------------------------------------------
***              This happens when two density matrices are convoluted
***              but the processes they belong to (e.g., production
***              and decay) do not match. This could happen if the
***              beam specification does not match the hard
***              process. Or it may indicate a WHIZARD bug.
******************************************************************************
******************************************************************************

[kilian]

Again, example?

[Juergen Reuter]

This seems to be a duplicate of what was posted in #495. There were way to many tickets.

[Juergen Reuter]

It was a duplicate.