Modifications to signal model machinery.

- Fix for global scales fudge factors
- Tweaks to datacard creator
- Set MH to const when reading an already existing model
- Option to disable syst plots
Merge pull request #413 from musella/topic_datacard_and_plots

Modifications to signal model machinery.
Merge pull request #3 from h2gglobe/master

Merge globe signal model tweaks