init_EinastoLens(D_d::RV=NaN, x_c::RV=0.0, y_c::RV=0.0, k_s::RV=NaN, x_s::RV=NaN, n::RV=-2.0)

Initialize an Einasto lens with the given parameters. The lens model can be initialized with either the concentration c or the scale radius x_s. If both are provided, c will be used to calculate x_s and the input x_s will be overwritten. The parameter n defines the slope of the density profile.