Fix ATLASRECTS-5772 FPE in trackCovMatrix
Rare FPEs were noticed in https://its.cern.ch/jira/browse/ATLASRECTS-5772, corresponding to cases where diagonal elements of the track covariance matrix could get negative and affecting the computation of the correlation coefficients used to stored the off-diagonal elements. This MR adds a protection against those cases by checking that the diagonal elements are positive first, setting the corresponding off-diagonal elements to zero otherwise.
Some refactoring of implementation for the compressed case is also implemented to use loops.
This was checked to fix the FPE error reported and to leave the track covariance matrix unaffected otherwise.