call enableExtrapolation() before h_.linkTo(p_) in YieldCurve

[haoxiaofeng]

Change enableExtrapolation() to be called before h_.linkTo(p_) so that the correct extrapolation_ value is available in h_'s observers

[mgroncki]

Hi Haoxiaofeng, thanks for your PR. I have difficulties understanding the issue here. Could you please share an example example illustrating a scenario where the old code behaves differently from the new version? Thanks a lot