Add helper function to store neighbor spacetime data in GhValenciaDivClean

[ncorsobh]

Proposed changes

Refactors duplicated code in the Filters.cpp and Derivatives.cpp files in GhValenciaDivClean/FiniteDifference. When upgrading GhValenciaDivClean to support higher order FD, the NeighborVariables in these files may include extra tags in some circumstances, so this is mostly to prevent future re-duplication of these same lines for NeighborVariables taking on different types.

Upgrade instructions

Code review checklist

• 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.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments