BUG-FIX: covmat was ill computed since the change to 'scale' parameter
BUG-FIX: covmat was ill computed since the change to 'scale' parameter

When taking the sigma for the covariance matrix, the code used to pass
the scale instead. Now amended.

miguelzuma/montepython_zuma,baudren/montepython_public,baudren/montepython_public,miguelzuma/montepython_zuma