Document CAFOS equations and substitute them with clearer expressions.

[juanep97]

These lines taken from IOP3 code are not very clear.

            Theta_0 = 0
        
            if Q_I >= 0:
                Theta_0 = math.pi 
                if U_I > 0:
                    Theta_0 = -1 * math.pi
                # if Q_I < 0:
                #     Theta_0 = math.pi / 2
                
            Theta = 0.5 * math.degrees(math.atan(U_I / Q_I) + Theta_0)
            dTheta = 0.5 * 180.0 / math.pi * (1 / (1 + (U_I/Q_I) ** 2)) * math.sqrt( (dU_I/Q_I)**2 + (U_I*dQ_I/Q_I**2)**2 )

I think they can be just substituted by the same expressions that DIPOL class uses.