Added general and specific path checking for yaml files (pydantic)

[Short description explaining the high-level reason for the pull request]

Additions

Added provision that checks all new entries added to configuration yaml file appearing to be pathways for actual existence in directory tree
Added specific verification for existence of all already-existing Paths in yaml file

[ ] PR has an informative and human-readable title
[ ] Changes are limited to a single goal (no scope creep)
[ ] Code can be automatically merged (no conflicts)
[ ] Code follows project standards (link if applicable)
[ ] Passes all existing automated tests
[ ] Any change in functionality is tested
[ ] New functions are documented (with a description, list of inputs, and expected output)
[ ] Placeholder code is flagged / future todos are captured in comments
[ ] Visually tested in supported browsers and devices (see checklist below :point_down:)
[ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
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