Issue #432: Remove exported filtering functions

[athowes]

Description

This PR closes #432.

It moves one filtering function into the two places it's used. And deletes the other which is used no where.

We could have follow up issue on adding back in some better simulation code about this, but we could also do that later.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• My code follows the established coding standards.
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[athowes]

@seabbs lmk if you want me to take back over on getting this passing tests

[seabbs]

I think this looks okay now