FT contributions: fix multiple exchanges from same key

[marc-vdm]

• fix FT Contributions mis-counting double exchanges from same key #1585

problem was with multiple exchanges from same key, where only one would be counted. Now all exchanges to/from same key are summed correctly.

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update the documentation
  • For in-code documentation, please follow the numpy style guide.
  • For user documentation, please update the wiki in
    ./activity_browser/docs/wiki
• Update tests.
• Link this PR to related issues by using closing keywords.

If you have write access (otherwise a maintainer will do this for you):

• Categorize the PR by setting a good title and adding one of the labels:
  bug, feature, ui, change, documentation, breaking, ci
  as they show up in the changelog.
• Add a milestone to the PR (and related issues, if any) for the intended release.
• Request a review from another developer.

[coveralls]

coverage: 53.153% (+0.01%) from 53.139%
when pulling f923da9 on marc-vdm:1585-FT-cont
into 5e093d0 on LCA-ActivityBrowser:main.