Messenger interface in nain4

[gonzaponte]

We need to provide an interface for G4GenericMessenger in nain4¹, probably with a build pattern to tweak the commands properly. Initial work in #14.

Important finding: when defining a new unit with G4UnitDefinition, the table of units must exist already, which Geant4 doesn't ensure. Our interface can do this automagically.

Footnotes

1. And keep an eye on the other ways to build messengers ↩