added documentation for using particular refdata files for unit testing

[jayantbhakar]

📝 Description

Type: 📝 documentation

This PR adds documentation to the developer workflow when working with unit tests. It says how to download only a required slice of refdata for running some unit tests of interest .

Also, link issues affected by this pull request by using the keywords: close, closes, closed, fix, fixes, fixed, resolve, resolves or resolved.

📌 Resources

Examples, notebooks, and links to useful references.

🚦 Testing

How did you test these changes?

• Testing pipeline
• Other method (describe)
• My changes can't be tested (explain why)

☑️ Checklist

• I requested two reviewers for this pull request
• I updated the documentation according to my changes
• I built the documentation by applying the build_docs label

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[codecov]

Codecov Report

Merging #2142 (7bab82a) into master (e3184f4) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2142      +/-   ##
==========================================
- Coverage   61.63%   61.60%   -0.04%     
==========================================
  Files          75       76       +1     
  Lines        8649     8681      +32     
==========================================
+ Hits         5331     5348      +17     
- Misses       3318     3333      +15     

Impacted Files	Coverage Δ
tardis/plasma/standard_plasmas.py	70.09% <0.00%> (-4.40%)	⬇️
tardis/plasma/properties/plasma_input.py	100.00% <0.00%> (ø)
tardis/plasma/properties/property_collections.py	100.00% <0.00%> (ø)
tardis/plasma/properties/rate_matrix_index.py	40.00% <0.00%> (ø)
tardis/io/util.py	80.50% <0.00%> (+2.51%)	⬆️

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