Lepton Meson HNL Anisotropies and HNL Polarization fix

[LuisPelegrina]

In this request two main issues in the HNL part of MeVPrtlGen have been tackled:

1. HNL Polarisation calculations have been revised:
   • Polarisation calculation are now also included in Tau decays into HNLs, using the same formula as the Kaon decays, and are implemented inside Tau2HNLFlux_tool.cc
   • A bug in the polarisation calculation in the Kaon case is now solved. K^{+}->l^{+}N and K^{-} -> l^{-N} where programmed to have the same polarisation instead of opposite. This now has been fixed inside the module Kaon2HNLFlux_tool.cc.
     2- The implementation of anisotropies in the l^{-/+}M^{+/-} decay is now implemented:
   • AnisotropicTwoBodyDecay.h and AnisotropicTwoBodyDecay.cpp have been added to handle anisotropic decay.
     • The main function is AnisotropicTwoBodyDist, which samples the integral that makes the decay anisotropic.
     • The integral sampled is given by: I = (1 -Pol) * I_{1}^{-}/2 + (1 + Pol) * I_{1}^{+)/2.
     • The function also changes the reference frame to correctly match the original formula in 1905.00284 in which the angle is given in respect to the HNL direction.
   • HNLMakeDecay_tool.cc has changed to include the anisotropic sampling. The boosting of the daughter particles is performed in this part of the code, the AnisotropicTwoBodyDist only gives the distribution in the HNL rest frame.
   • hnl.fcl (the config fhicl for HNL decays) has been changed to include a new boolean: DecayIsTwoBodyAnisotropic. This boolean lets the user choose between the old implementation (done by Gray) and the new one. It is turned true by default.

This pull request is linked to sbndcode pull request with link:
SBNSoftware/sbndcode#657

[gputnam]

Looks good! Made some comments about configuration.

[kjplows]

trigger build

[FNALbuild]

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✔️ CI build for LArSoft Succeeded on slf7 for e26:prof -- details available through the CI dashboard

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❌ CI build for ICARUS Failed at phase build ICARUS on slf7 for c14:prof -- details available through the CI dashboard

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[FNALbuild]

❌ CI build for ICARUS Failed at phase build ICARUS on slf7 for e26:prof -- details available through the CI dashboard

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[FNALbuild]

⚠️ CI build for SBND Succeeded with warning at phase build on slf7 for e26:prof - ignored warnings for build -- details available through the CI dashboard

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