Complex functions fix

[OrlowskiWojtek]

When trying to perform fit on complex function (returning complex value) there is no method for max and min between complex value and float value. My code:

function fit_arx(freqs, T::Vector{ComplexF64})
    
    function arx_model(x,p)
        return @. 1 / ((1 + 1im* x / p[1]) * (1 + 1im* x / p[2]))
    end

    p0 = [1.95 + 0im, 20.2 + 0im]

    curve_fit(arx_model, freqs, T, p0)    
end

resulting a bug:
ERROR: MethodError: no method matching isless(::Float64, ::ComplexF64)...

possible fix:
in levenberg_marquardt.jl in increase and dreacre of trust region radius (lines 262 and 266) switch

            if rho > good_step_quality
                # increase trust region radius
                lambda = max(lambda_decrease * lambda, MIN_LAMBDA)
            end
        else
            # decrease trust region radius
            lambda = min(lambda_increase * lambda, MAX_LAMBDA)
        end

to

            if rho > good_step_quality
                # increase trust region radius
                lambda = max(abs(lambda_decrease * lambda), MIN_LAMBDA)
            end
        else
            # decrease trust region radius
            lambda = min(abs(lambda_increase * lambda), MAX_LAMBDA)
        end