model.exogenous.Sigma obsolete for new MvNormal, change it to Greek

[ezgioz]

Using the perturb function (for BGM) I received the following error message:

• 'Normal' object has no attribute 'Sigma'

I changed model.exogenous.Sigma to model.exogenous.Σ

@albop Does it seem coherent for the rest of the code?

[albop]

Yes it makes sense. Thanks for the fix. Can you remve the old line instead of commenting it ?

[ezgioz]

@albop done