Adding missing import for IonNumberDensityHeNLTE in standard_plasmas.py

[DhruvSondhi]

This PR aims to fix a missing import present in the standard_plasmas.py. Added the import for IonNumberDensityHeNLTE from properties.py.
Fix #1725

Description

Motivation and context

How has this been tested?

• Testing pipeline.
• Other.

Type of change

• Bug fix.
• New feature.
• Breaking change.
• None of the above.

Checklist

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
  • (optional) I have built the documentation on my fork following the instructions.
• I have assigned and requested two reviewers for this pull request.

[tardis-bot]

Before a pull request is accepted, it must meet the following criteria:

• Is the necessary information provided?
• Is this a duplicate PR?
  • If a new PR is clearly a duplicate, ask how this PR is different from the original PR?
  • If this PR is about to be merged, close the original PR with a link to this new PR that solved the issue.
• Does it pass existing tests and are new tests provided if required?
  • The test coverage should not decrease, and for new features should be close to 100%.
• Is the code tidy?
  • No unnecessary print lines or code comments.

[codecov]

Codecov Report

Merging #1722 (11d270a) into master (c6d8b91) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1722   +/-   ##
=======================================
  Coverage   61.87%   61.87%           
=======================================
  Files          63       63           
  Lines        5852     5852           
=======================================
  Hits         3621     3621           
  Misses       2231     2231           

Impacted Files	Coverage Δ
tardis/tardis/plasma/standard_plasmas.py	73.11% <0.00%> (ø)

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[marxwillia]

Good catch.