functions to get derivs of 3-metric from GH variables

[prayush]

Proposed changes

Types of changes:

• Bugfix
• New feature

Component:

• Code
• Documentation
• Build system
• Continuous integration

Code review checklist

• The PR passes all checks, including unit tests, clang-tidy and IWYU.
  For instructions on how to perform the CI checks locally refer to the Dev
  guide on the Travis CI.
• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.

Further comments