Fix low temperature problem of Van Regemorter approximation

[chvogl]

The exponential in the calculation of the van Regemorter approximation can become infinite, e.g. for 100 K and transition energies bigger than roughly 6.5 eV:
np.exp(((6.5 * units.eV) / (100 * units.K * const.k_B)).to(''))
The product of the exponential integral E1 and the exponential, however, stays finite. I have added an implementation of
the product that does not calculate the exponential for large values but instead uses the Laurent series expansion.

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.

[codecov]

Codecov Report

Merging #1992 (91856bb) into master (ae19f26) will increase coverage by 0.04%.
The diff coverage is 92.85%.

❗ Current head 91856bb differs from pull request most recent head e048c6c. Consider uploading reports for the commit e048c6c to get more accurate results

@@            Coverage Diff             @@
##           master    #1992      +/-   ##
==========================================
+ Coverage   59.99%   60.04%   +0.04%     
==========================================
  Files          70       70              
  Lines        8111     8121      +10     
==========================================
+ Hits         4866     4876      +10     
  Misses       3245     3245              

Impacted Files	Coverage Δ
tardis/plasma/properties/atomic.py	56.17% <88.88%> (+1.07%)	⬆️
tardis/io/atom_data/base.py	89.71% <100.00%> (+0.17%)	⬆️
tardis/plasma/standard_plasmas.py	74.48% <100.00%> (+0.26%)	⬆️

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[Rodot-]

Please provide new tests for these class methods to verify and guarantee their behavior. The test should verify that 500 is a good upper limit for the scipy/numpy calculation.