Add virtual photon linear polarization and virtual photon flux to inclusive kinematics collection.

[simonge]

I wanted to gather opinions on extending the Inclusive kinematics collection to include virtual photon polarisation and flux. Reporting on these directly with the other kinematics will help simplify some of the downstream physics studies.

Where linear polarization can be given by:
$$P_\gamma=1/(1+2(1+E_{\gamma}^2/Q^2)\cdot\tan(\theta_{e^-_{scat}})^2)$$

And virtual photon flux:
$$\Gamma_{\gamma^*}=\frac{1}{137} \cdot \frac{1}{4\pi^2} \cdot \frac{1}{ E_{e_{beam}^-} \cdot E_{e_{scat}^-} } \cdot \frac{W^2-m_{target}}{m_{target}} \cdot \frac{1}{Q^2\cdot(1-P_\gamma)}$$

Translated (probably incorrectly) from code provided by @dglazier :

Double_t GammaPol(){return 1./(1+2*(1+GammaE()*GammaE()/Q2())*TMath::Tan(ElScatTh()/2)*TMath::Tan(ElScatTh()/2));}
Double_t GammaFlux(){return(1./137/4/TMath::Pi()/TMath::Pi())/fElin.E()*fElsc.E()*((W()*W()-fTar.M2())/fTar.M())/Q2()/(1-GammaPol()); }