Improved default number of interpolate_shells for the formal integral

[chvogl]

Description

This PR changes the default value of interpolate_shells for the formal integral to a more sensible value: twice the number of computational shells (but at least 80).

Motivation and Context

The current default value of interpolate_shells (-1, i.e. no interpolation) for the formal integral yields integrated spectra of bad quality, which do not agree with the real and virtual spectrum.

How Has This Been Tested?

• Testing pipeline
• Reference Data Comparison following these instructions
• On my local machine (for now)

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• None of the above (please describe)

Checklist:

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have built the documentation on my fork following these instructions
• I have assigned and requested two reviewers for this pull request

[chvogl]

What do people think? Is this an improvement over the current default behaviour?

[codecov]

Codecov Report

Merging #1512 (afbaa6b) into master (6816e21) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head afbaa6b differs from pull request most recent head d175e13. Consider uploading reports for the commit d175e13 to get more accurate results

@@            Coverage Diff             @@
##           master    #1512      +/-   ##
==========================================
- Coverage   69.08%   69.06%   -0.02%     
==========================================
  Files          68       68              
  Lines        6100     6106       +6     
==========================================
+ Hits         4214     4217       +3     
- Misses       1886     1889       +3     

Impacted Files	Coverage Δ
tardis/tardis/montecarlo/formal_integral.py	54.76% <0.00%> (-0.23%)	⬇️
...rdis/tardis/montecarlo/montecarlo_numba/vpacket.py	17.47% <0.00%> (-0.18%)	⬇️
tardis/tardis/plasma/properties/ion_population.py	86.27% <0.00%> (+0.13%)	⬆️

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