Bugfix for FluxWeight and MeVPrtl

[VCLanNguyen]

1. Fix a bug in MeVPrtl: BNBKaonGen incorrectly label boone ntuple as dk2nu format. Updated to it simple flux format. This would prevent Flux Weight tools to to crash when inputting MeVPrtl files.
2. Add a feature in Flux Weight tools: to pick up neutrino energy from MCTruth instead if flux.nenergyn == -1.
   In MeVPrtl, by default, MCFlux neutrino energy is filled as -1 since not every meson in flux file would result in a neutrino. It instead computes an equivalent neutrino energy and saves it as MCTruth.
   This change should not impact the neutrino workflow since every meson results in a neutrino, and MCFlux flux.nenergyn should always be filled.

Description

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