Check if atom data download exists

[wkerzendorf]

and do not redownload unless forced.

[tardis-bot]

*beep* *bop*

Hi, human.

The docs workflow has succeeded ✔️

Click here to see your results.

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (0599113) 67.17% compared to head (b210089) 67.16%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
tardis/io/atom_data/atom_web_download.py	14.28%	6 Missing ⚠️
tardis/io/configuration/config_internal.py	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2479      +/-   ##
==========================================
- Coverage   67.17%   67.16%   -0.02%     
==========================================
  Files         156      156              
  Lines       13763    13767       +4     
==========================================
+ Hits         9245     9246       +1     
- Misses       4518     4521       +3     

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[andrewfullard]

Hopefully this helps the docs issue