WIP: Added spack packaging for pythia8

• This will add the variant hepmc3 to pythia8

IMO this was far too much work to add a missing configure flag.

containers/builder/spack.yaml

@@ -16,8 +16,8 @@ spack:
           +fftw +fortran +gdml +http +mlp +pythia8 
           +root7 +tmva +vc +xrootd +ssl ^mesa+opengl 
           -llvm -osmesa
-    - pythia8@8303
     - hepmc3@3.2.2 +python +rootio 
+    - pythia8@8303
     - stow@2.3.1
     - cairo+fc+ft+X+pdf+gobject
     - podio@master