xAOD::TrackParticle Covariance Matrix Evolution, 21.2 branch (2019.08.08.)
This is meant as a replacement for !24240 (closed) and has been built on top of it.
Following the discussions with the tracking and reconstruction experts, this is an attempt at implementing a schema evolution for xAOD::TrackParticle. In the new setup the covariance matrix of the defining parameters end up in two separate variables called "definingParametersCovMatrixDiag" and "definingParametersCovMatrixOffDiag". Making it possible to handle the diagonal and off-diagonal elements of the covariance matrix more easily in the I/O system.
The stored elements in "definingParametersCovMatrixOffDiag" are not directly the off-diagonal elements of the covariance matrix but rather the correlation coefficients between -1 and 1, which are better suited for subsequent lossy compression.
The xAODMaker::TrackParticleCompressorTool is also modified to optionally drop some of the off-diagonal elements (assumed to be uncorrelated).
A check requiring a positive covariance matrix determinant has also been added, as negative values were found to cause errors while rerunning FTAG vertex refitting in derivations.
Pinging @akraszna and @christos for them to comment if needed