Global configuration for electron scattering parameters

[tejinc]

Currently, the electron scattering cross-section is hard coded via the coupling constants (g1 g2). It might be useful to have user-defined parameters because.

1. global fit values of the Weinberg angle may change as more measurements were made.
2. the electron scattering cross-section will change depending on if we want to take into account radiative correction (https://journals.aps.org/prd/pdf/10.1103/PhysRevD.101.032002)
   A more general (and yet simple for users) way to specify the coupling constants would be highly useful.