tauRecTools: use the right equation to calculate combined sigma

Xiaozhong Huangrequested to merge
xiaozhon/athena:resolution into master

We combine the four momentum of two calibrations in CombinedP4FromRecoTaus. The problem is that the calculation is not right for the combined sigma. It should be sigma(combined) = sigma_1^2 * sigma_2^2 * (1 - rho^2) / (sigma_1^2 + sigma_2^2 - 2 * pho * sigma_1 * sigma_2)

Merge request reports