Bug fix: covariance matrix input handling

when given an input covariance matrix, 1-sigma errors should be used
for the other varying parameters with no other information. It now
acts like that again.
Bug fix: behaviour for no varying parameters

when called with no varying parameters, Monte Python should call the
cosmological module and evaluate the likelihood of the desired
experiments for these fixed values. This bug was instead using default
Class values.
Note that this was only affecting the behaviour where every parameters
were fixed, not when only some of them were.