Fix freezing functionality for constraint nuisances; modify external postfit functionality; update likelihood scan

[davidwalter2]

• The freezing parameter functionality is now also used for the likelihood scans
  • Previously the gradient on the frozen parameters were not stopped for the constraint term, this is now fixed by stopping the gradients in the get_blinded_theta() and get_blinded_poi() functions
• The fit functionality was moved to a function commonly used for the nominal fit and likelihood scan
• Plotting script for likelihood scan is modified as well