Covariances between betas for all options in ProbABEL

[lckarssen]

This is a 'liftover' of feature request #1337 from the R-forge tracker.

see ProbABEL/doc/CHANGES.LOG:

... This "plus" version reports naive covariance (default) or robust covariance
(option --robust) between b_SNP and b_covX_SNP estimates, for palinear and
palogist (can NOT report covariance for pacoxph, Cox regression).
Covariances are no reported for options --score, --allcov, --mmscore

and http://forum.genabel.org/viewtopic.php?f=6&t=231