methods don't update

[rpoleski]

There's a bug in v3 MagnificationCurve - methods don't update:

    t_vec = np.array([3.5, 2., 1., 0.5, 0.])
    params_0 = mm.ModelParameters({'t_0': 0., 'u_0': 0.5, 't_E': 1., 'rho': 1.})
    mag_curve_0 = mm.MagnificationCurve(times=t_vec, parameters=params_0)
    mag_curve_0.set_magnification_methods([-5., 'finite_source_uniform_Lee09', 5.], 'point_source')
    print(mag_curve_0.methods_indices)
    mag_curve_0.set_magnification_methods([-5., 'finite_source_uniform_WittMao94', 5.], 'point_source')
    print(mag_curve_0.methods_indices)

Returns:

{'finite_source_uniform_Lee09': array([ True,  True,  True,  True,  True])}
{'finite_source_uniform_Lee09': array([ True,  True,  True,  True,  True])}

Expected:

{'finite_source_uniform_Lee09': array([ True,  True,  True,  True,  True])}
{'finite_source_uniform_WittMao94': array([ True,  True,  True,  True,  True])}

@jenniferyee