Fix thermal models

[andreufont]

The order of sigma_T parameters was for some reason different. I'll check that this does not affect the forecasts, and merge the pull request.

I've also used this pull request to do some minor changes that I will document as well.

[andreufont]

Other changes:

• added option to provide starting point in iminuit minimizer
• updated notebooks to play with emulator covariance
• added notebooks to play with nuisance models