Adapt GVA formula to exclude FDI retention projects

[oliverjwroberts]

Description of change

This PR modifies how Gross Value Added (GVA) is calculated for an investment project. Now, projects with an FDI type of retention will return a GVA multiplier of None, and thus, have no GVA.

See ticket CLS2-788 for more information.

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.39%. Comparing base (96c5b07) to head (1150117).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5377   +/-   ##
=======================================
  Coverage   96.38%   96.39%           
=======================================
  Files         930      930           
  Lines       22065    22068    +3     
  Branches     1970     1971    +1     
=======================================
+ Hits        21267    21272    +5     
+ Misses        653      652    -1     
+ Partials      145      144    -1     

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