Allow for optional saving of background state

[n-claes]

PR description

This PR adds the option to disable saving the background to the datfile, which is useful to reduce the datfile size for high resolution parameter studies. Note that eg. continua, sound speed... can not be calculated in this case, appropriate errors will be thrown by Pylbo. This change modifies the datfile but is backwards compatible.

New features

Legolas

• New option write_background = .false. in the savelist, which disables saving the background to the datfile.

Checklist

• all tests pass (and for new features, new tests are added)
• documentation has been updated (if applicable)

[codecov]

Codecov Report

Merging #136 (cf185e2) into develop (e98d984) will increase coverage by 0.03%.
The diff coverage is 97.10%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #136      +/-   ##
===========================================
+ Coverage    90.65%   90.68%   +0.03%     
===========================================
  Files          158      158              
  Lines         8899     8952      +53     
===========================================
+ Hits          8067     8118      +51     
- Misses         832      834       +2     

Flag	Coverage Δ
legolas	93.11% <83.33%> (-0.03%)	⬇️
pylbo	86.33% <100.00%> (+0.20%)	⬆️

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