Change QC difference plot colorbar to scientific notation

[mattdturner]

PR checklist

• Short (1 sentence) summary of your PR:
  Change the colorbar in the difference plot (for the QC script) to scientific notation.
• Developer(s):
  @mattdturner
• Suggest PR reviewers from list in the column to the right.
• Please copy the PR test results link or provide a summary of testing completed below.
  No testing necessary, as this only touches the QC script. Tested and confirmed that the QC script still works and provides identical plots (with the exception of colorbar units) to master.
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on Icepack or any other models?
  • Yes
  • No
• Does this PR add any new test cases?
  • Yes
  • No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/. A test build of the technical docs will be performed as part of the PR testing.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please provide any additional information or relevant details below:
  This PR addresses Improve colormap ticks for QC test generated figures  #501

[mattdturner]

[phil-blain]

Thanks @mattdturner !