Updates on nll scans

[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 externalPostfit functionality is slightly modified such that
  • the beta parameters are profiled after loading
  • it can be used as a starting point in a fit

on top of #95