moved helium nlte to a seperate file helium_nlte.py

[sonachitchyan]

📝 Description

Type: 🪲 bugfix | 🚀 feature | ☣️ breaking change | 🚦 testing | 📝 documentation | 🎢 infrastructure

This PR moves helium_nlte to a separate file for better organizing of nlte part of the code.

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

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☑️ 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 #2130 (e9e0ce5) into master (a12b983) will increase coverage by 0.03%.
The diff coverage is 34.37%.

@@            Coverage Diff             @@
##           master    #2130      +/-   ##
==========================================
+ Coverage   61.60%   61.63%   +0.03%     
==========================================
  Files          74       75       +1     
  Lines        8641     8649       +8     
==========================================
+ Hits         5323     5331       +8     
  Misses       3318     3318              

Impacted Files	Coverage Δ
tardis/plasma/properties/nlte.py	100.00% <ø> (+59.43%)	⬆️
tardis/plasma/properties/helium_nlte.py	34.37% <34.37%> (ø)

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