ECC-2035: Revamped directory structure and introduction of CamelCase in accessors

[joobog]

• JIRA: https://jira.ecmwf.int/browse/ECC-2035
• Re-organise directory structure. Move ./src to ./src/eccodes
• Use CamelCase in accessors

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.60%. Comparing base (60f74ab) to head (f8b16c3).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #296      +/-   ##
===========================================
- Coverage    87.87%   87.60%   -0.28%     
===========================================
  Files          841      843       +2     
  Lines        62006    62195     +189     
  Branches     11061    11109      +48     
===========================================
- Hits         54488    54484       -4     
- Misses        7518     7711     +193     

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