Add abstract base classes

[alexamici]

Related to #243

[codecov-commenter]

Codecov Report

Merging #246 (2830e87) into from-stream-to-file (2fa6d24) will decrease coverage by 0.05%.
The diff coverage is 98.91%.

@@                   Coverage Diff                   @@
##           from-stream-to-file     #246      +/-   ##
=======================================================
- Coverage                96.37%   96.32%   -0.06%     
=======================================================
  Files                       25       26       +1     
  Lines                     1793     1822      +29     
  Branches                   211      213       +2     
=======================================================
+ Hits                      1728     1755      +27     
- Misses                      39       40       +1     
- Partials                    26       27       +1     

Impacted Files	Coverage Δ
cfgrib/abc.py	97.26% <97.26%> (ø)
cfgrib/cfmessage.py	95.45% <100.00%> (ø)
cfgrib/dataset.py	98.96% <100.00%> (-0.01%)	⬇️
cfgrib/messages.py	91.42% <100.00%> (-1.53%)	⬇️
tests/test_10_cfunits.py	100.00% <100.00%> (ø)
tests/test_20_cfcoords.py	100.00% <100.00%> (ø)
tests/test_20_main.py	100.00% <100.00%> (ø)
tests/test_20_messages.py	99.27% <100.00%> (+<0.01%)	⬆️
tests/test_25_cfmessage.py	100.00% <100.00%> (ø)
tests/test_30_dataset.py	100.00% <100.00%> (ø)
... and 9 more

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