updating test case and code to read forcings with '.csv' extension

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

Additions

[ ] 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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