Introduce pre-defined keys for MC event header info

A number of keys into the ME event header information
mapping is defined.   This is to ensure that code will
use the same keys when ever information is set.

Additional, non-predefined keys, are still possible.

This makes it much more robust when we ask for specific
MC information from the event header, such as

- cross-section(s)
- weight(s)
- Heavy-ion "geometry" parameters
  - Npart in projectile and target
  - Ncoll in various views
    - Overall
    - Hard
    - wounded-nucleon on nucleon
    - nucleon on wounded-nucleon
    - wounded on wounded
- Parton distribution function parameters

This is crucial for building a HepMC event structure which
can be passed on to say for example Rivet analyses.

Author: cholmcc

DataFormats/simulation/include/SimulationDataFormat/MCEventHeader.h

@@ -27,6 +27,68 @@ namespace dataformats
 
 class GeneratorHeader;
 
+  /** Common keys for information in MC event header */
+  struct MCInfoKeys {
+    /** @{
+	@name HepMC3 heavy-ion fields */
+    static constexpr const char* impactParameter        = "Bimpact";
+    static constexpr const char* nPart                  = "Npart";
+    static constexpr const char* nPartProjectile        = "Npart_proj";       
+    static constexpr const char* nPartTarget            = "Npart_targ";
+    static constexpr const char* nColl                  = "Ncoll";
+    static constexpr const char* nCollHard              = "Ncoll_hard";
+    static constexpr const char* nCollNNWounded         = "NColl_NNw";
+    static constexpr const char* nCollNWoundedN         = "NColl_NwN";
+    static constexpr const char* nCollNWoundedNwounded  = "NColl_NwNW";
+    static constexpr const char* planeAngle             = "eventPsi";
+    static constexpr const char* sigmaInelNN            = "sigmaInelNN";
+    static constexpr const char* centrality             = "centrality";
+    static constexpr const char* nSpecProjectileProton  = "Nspec_proj_p";
+    static constexpr const char* nSpecProjectileNeutron = "Nspec_proj_n";
+    static constexpr const char* nSpecTargetProton      = "Nspec_targ_p";
+    static constexpr const char* nSpecTargetNeutron     = "Nspec_targ_n";
+    /** @} */
+    /** @{
+	@name HepMC3 PDF information
+
+	In principle a header can have many of these.  In that case,
+	each set should be prefixed with "_<X>" where "<X>" is a
+	serial number.
+    */
+    static constexpr const char* pdfParton1Id           = "pdf_parton_1_id";
+    static constexpr const char* pdfParton2Id           = "pdf_parton_2_id";
+    static constexpr const char* pdfX1                  = "pdf_x1";
+    static constexpr const char* pdfX2                  = "pdf_x2";
+    static constexpr const char* pdfScale               = "pdf_scale";
+    static constexpr const char* pdfXF1                 = "pdf_par_x1"; 
+    static constexpr const char* pdfXF2                 = "pdf_par_x2";
+    static constexpr const char* pdfCode1               = "pdf_lhc_1_id";
+    static constexpr const char* pdfCode2               = "pdf_lhc_2_id";
+    /** @} */
+    /** @{
+	@name HepMC3 cross-section information
+
+	In principle we can have one cross section per weight. In that
+	case, each should be post-fixed by "_<X>" where "<X>" is a
+	serial number.  These should then matcht possible names of
+	weights.
+    */
+    static constexpr const char* acceptedEvents  = "accepted_events";
+    static constexpr const char* attemptedEvents = "attempted_events";
+    static constexpr const char* xSection        = "cross_section"; 
+    static constexpr const char* xSectionError   = "cross_section_error";
+    /** @} */
+    /** @{
+	@name Common fields */
+    static constexpr const char* generator        = "generator";
+    static constexpr const char* generatorVersion = "version";
+    static constexpr const char* processName      = "processName";
+    static constexpr const char* processCode      = "processCode";
+    static constexpr const char* weight           = "weight";
+    /** @} */
+      
+  };
+
 /*****************************************************************/
 /*****************************************************************/