Return periodic torsion params, regardless of k value

[umesh-timalsina]

PR Summary:

See #470 for further details. Previously, we didn't return any parameters if the value of k == 0. This PR attempts to fix that. Resolves #470.

PR Checklist

---

• [x] Includes appropriate unit test(s)
• [x] Appropriate docstring(s) are added/updated
• [x] Code is (approximately) PEP8 compliant
• [x] Issue(s) raised/addressed?

[codecov[bot]]

Codecov Report

Merging #471 (e58a2ac) into master (39fccf6) will decrease coverage by 0.01%. The diff coverage is 100.00%.

:exclamation: Current head e58a2ac differs from pull request most recent head c3de41f. Consider uploading reports for the commit c3de41f to get more accurate results

@@            Coverage Diff             @@
##           master     #471      +/-   ##
==========================================
- Coverage   73.82%   73.80%   -0.02%     
==========================================
  Files          17       17              
  Lines        1849     1848       -1     
==========================================
- Hits         1365     1364       -1     
  Misses        484      484              

[umesh-timalsina]

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[justinGilmer]

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[umesh-timalsina]

/azp run

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