Acre unit support (HIP-RA-X reservoir area) [v3.15.3]

[softwareengineerprogrammer]

Description

Support acre units - relevant to HIP-RA-X reservoir area

Testing & Verification

1. Unit tests added: test_area_acre
2. Unit tests updated: N/A
3. Manual testing:
   1. Manually tested
   2. Web interface:
      1. 

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Change Checklist

Observe the following checklist items before and after merging, as applicable.

Before merging:

1. I certify that I am a human and I have manually verified the correctness of the changes in this PR.
2. Add a CHANGELOG entry: 7587c7d

After merging:

1. Push tag(s) to GitHub: git push fork refs/tags/v3.15.3: https://github.com/softwareengineerprogrammer/GEOPHIRES/releases/tag/v3.15.3

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Self-reviewed in #176