Add vcov functionality

[tombisho]

In standard R with the survival package one can get the variance-covariance matrix using vconv()

In a future version of dsSurvival this should be added. If the model is stored on the server, then it is a case of running vcov() on that model and returning the results.

If the model itself has passed the disclosure checks, then it should be OK to release these summary statistics.

[neelsoumya]

changes made by @tombisho