Add functions to fit models to return DP likelihood(s) and signal DP amplitudes

Summary:

• Add functions to fit models that return:
  • signal DP amplitudes
  • signal and background DP likelihood(s)

• Add example to plot histograms of likelihood values

Caveat: SCF signal is not currently handled - an appropriate warning is printed

Closes T227

Test Plan: Ran example and histogram matched expectation

Reviewers: jback

Reviewed By: jback

Tags: Laura

Maniphest Tasks: T227

Differential Revision: https://phab.hepforge.org/D104