Updated xsec and br numbers, including br errors

For now, the xs*br errors are summed in quadrature. Although, really, the errors should be treated as independent nuisance parameters.

Also, in the new version of br, there aren't any values for ss. The old version is left in there in case it's needed. It can be accessed by sending in the keyword: brversion = 'v1' in the xsbr() function.