Yield Curves: Tullet chain symbols (improvement)

Tullet chain symbols can now be added as individual values in the "Yield Curves" widget.

Examples: 11886SECEURL.MRKTC.10Y, BMKGB01Y.TFI, SBSUSD.IIRSE.15Y, BMKNZ02Y.TFI, BMKNZ20Y.TFI