Abstract away the definition of the input product

[alexamici]

Addresses: #41

[codecov-commenter]

Codecov Report

Merging #45 (b1c8779) into main (84b62df) will increase coverage by 3.32%.
The diff coverage is 73.93%.

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   70.37%   73.69%   +3.32%     
==========================================
  Files          17       21       +4     
  Lines         648      749     +101     
  Branches       54       59       +5     
==========================================
+ Hits          456      552      +96     
- Misses        184      187       +3     
- Partials        8       10       +2     

Impacted Files	Coverage Δ
tests/test_50_apps.py	53.84% <7.69%> (-11.54%)	⬇️
sarsen/__main__.py	41.17% <12.50%> (-3.99%)	⬇️
sarsen/apps.py	10.52% <18.18%> (-16.75%)	⬇️
sarsen/datamodel.py	70.00% <70.00%> (ø)
sarsen/sentinel1.py	84.53% <84.53%> (ø)
sarsen/__init__.py	100.00% <100.00%> (ø)
sarsen/radiometry.py	33.33% <100.00%> (+2.29%)	⬆️
tests/test_10_datamodel.py	100.00% <100.00%> (ø)
tests/test_20_sentinel1.py	100.00% <100.00%> (ø)
... and 1 more

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