[Rivet] [HepData] BUG fix!! HEpData -> Yoda problem: Crash for HepData with large (asymmetric) pT bin

[Graeme Watt]

Dear Kristin,

Thanks for finding this bug!  I looked at the HepData code that writes
the Yoda format and indeed the "xerr-" and "xerr+" values were being
written in the reverse order to what was indicated in the comment "#
xval   xerr-   xerr+   yval   yerr-   yerr+".  It is now fixed.

Best regards,
Graeme Watt (HepData)