Very Draft: Picker24/feature/user projections

[luketpickering]

use like:

  samplePDFDUNEBeamFD::AddProjection(
      "mysillyvar", [](dunemc_base const &dunemcSample, int iEvent) {
        return (dunemcSample.rw_erec_shifted[iEvent] -
                dunemcSample.rw_erec_lep[iEvent]) /
               (dunemcSample.true_q0[iEvent]);
      });

static function on the analysis subclass (as it needs to know the 'structs type'), functions need to be registered before being instantiated by the factory

then in config:

Binning:
  Axes:
    - VarStr: "ELepRec" 
      Uniform: [25,0,5]
      Title: "E_{lep.}^{Rec} [GeV]"
    - VarStr: "mysillyvar" 
      Uniform: [25,0,5]
      Title: "dumbdumb"