Add class for photon emission in hadronic phase
Merge branch 'master' into gsl_integration
change parameters.h to Physicalconstants.h in all the cpp files
add functions to HG_2to2_Scattering class to calculate photon emission rates
add gsl integration routines
fixed a bug in E2 integrand
Merge branch 'master' of https://github.com/chunshen1987/viscous_emissionrate_tables_HG_2to2_with_chemical_potential_and_Formfactor into gsl_integration
categorize the functions into class
Merge branch 'fixedPointsGaussIntegrations'