Populate COVAR2 coarse covariance matrix

The coarse 10x10 covariance matrix, COVAR2, of the 1-D spectrum output from the 2D DRP is defined in https://github.com/Subaru-PFS/datamodel/blob/master/datamodel.txt and is used to model the spectrophotometric errors.

As part of the flux calibration activities, generate COVAR2 and populate the corresponding pfsObject HDU with that information.