bug in (t_eff, u_0) parameterization for parallax models with negative u_0

[rpoleski]

@przemekmroz informed me that he found bug in parallax calculation for model with t_E calculated from t_eff and u_0 for negative u_0.
Trajectory calls ModelParameters.t_E for tau calculation. For this parameterization, ModelParameters.t_E returns self.t_eff / self.u_0, which is negative. We should add math.abs() in denominator or make a similar change. Other similar cases should be considered (e.g., t_E_1/2 for binary source models).

[rpoleski]

@przemekmroz - issue solved.
I'm not closing this thread because there could be other similar bugs that should be corrected.