add option to load sun data on the fly

[peterdudfield]

Pull Request

Description

add option to make sun data on the fly
Fixes #675

How Has This Been Tested?

added a unittest

• Yes

Checklist:

• My code follows OCF's coding style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked my code and corrected any misspellings

[codecov-commenter]

Codecov Report

Merging #676 (8c0dedf) into main (fdffacf) will increase coverage by 0.00%.
The diff coverage is 96.66%.

@@           Coverage Diff           @@
##             main     #676   +/-   ##
=======================================
  Coverage   93.42%   93.42%           
=======================================
  Files          48       48           
  Lines        3316     3332   +16     
=======================================
+ Hits         3098     3113   +15     
- Misses        218      219    +1     

Impacted Files	Coverage Δ
...asting_dataset/data_sources/sun/sun_data_source.py	97.43% <96.42%> (-1.01%)	⬇️
nowcasting_dataset/config/model.py	94.17% <100.00%> (+0.05%)	⬆️

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