Introduce PDG code enumeration

[mattiaisgro]

This pull request introduces a PDG code enumeration PDGCode which can be used as an integer value to reference particles by name. For example:

if (in[j] ==  21 || in[j] == 15)
      result[j] = 1;

becomes

if (in[j] == PDG_GLUON || in[j] == PDG_TAU)
      result[j] = 1;

The PDG codes are defined in the MCParticleCodes.h header file, which has no dependencies. Currently, only basic particles have been defined, but more may be added when needed.

Advantages:

• Improves code readability
• Reduces human error
• By using an unscoped enumeration, legacy code does not need to change. Software already using FCCAnalyses will not need to change, but new code may use this feature