mainly esthetic changes to sweighter code and some minor code refactoring

Merge Request Title: Minor Updates to the sFit code

Motivation

Mainly Blaise being picky with esthetics.

MR description

Minor changes to the executable to amend the D0_M range & cosmetics.

How-to

TBD, will be included in fit snakefile.

@mkenzie data has been re-run

• please can you use the data files that live in: /r01/lhcb/Bc2D0MuNuX/pipeline/nominal/DATA/fit/D0MuNu/2018;
• please let's continue with v3 of the MC tuples, as these are not PID-corrected. These may be found here: /r01/lhcb/Bc2D0MuNuX/pipeline/nominal/MC/fit/D0MuNu/2016.
• I suspect a cut on D0_M at +/- 60 MeV might be needed on the MC samples to bring them in line with the data produced in v4.

Clarity

Please provide an evaluation of the clarity level of the documentation and code (specifically, is the code commented? are the variables self-explanatory?). Please tick the level of clarity of code:

• Bare: README.md and alike are missing. The code is not very well documented. Please try to avoid merging if the code is in this state and spend some time documenting it; this will pay off in the long run.
• Acceptable: any other analyst should be able to understand what is being implemented by this MR with a couple of questions. Code has some comments and variables are reasonably named.
• Complete: README.md exists, code is commented, headers are in place and variables are self-explanatory. I have spent time making sure that this is very clear and ready out of the box. This is the gold standard.

Checks

Fit converges (assert(minuit.converge/hessin pos def not forced) statements added) + pulls from plots.

Check-list

To ensure clear documentation and comprehensive tracking of developments, please complete the following actions:

• Assign label(s)
• Assign milestone
• CC relevant developers

Before submitting MR:

• Branch has been rebased. Expect any conflicts in the MR to stem from alterations implemented by the author in this branch and not from a branch that is not up-to-date with uncorrelated changes in master. If this is not clear, please refer to this link to understand the advantages of rebasing a branch: rebasing on gitlab
• Assign reviewer(s)
• Code clarity evaluation performed.
• If code clairity level is Bare: opened issue and documented the steps needed to improve upon the clarity level.
• All sanity checks have passed. Attached plots and numbers to demonstrated this.
• Opened issue to outline how does the ANA need to describe the accomplished improvements implemented with this MR.

/cc @bldelane