Modernisation of accessor classes

[joobog]

We simplified the accessor classes by merging the data and functionality into one class, eliminating the need for separate classes.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.64426% with 334 lines in your changes missing coverage. Please review.

Project coverage is 87.75%. Comparing base (d360288) to head (7ee915b).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ccessor_class_data_apply_boustrophedonic_bitmap.cc	53.75%	37 Missing ⚠️
...essor/grib_accessor_class_data_g22order_packing.cc	81.87%	29 Missing ⚠️
...cessor/grib_accessor_class_data_complex_packing.cc	76.63%	25 Missing ⚠️
...ss_data_g1second_order_general_extended_packing.cc	87.05%	22 Missing ⚠️
src/accessor/grib_accessor_class_codetable.cc	87.50%	19 Missing ⚠️
.../grib_accessor_class_data_apply_boustrophedonic.cc	76.66%	14 Missing ⚠️
src/accessor/grib_accessor_class_dictionary.cc	75.47%	13 Missing ⚠️
...essor/grib_accessor_class_data_jpeg2000_packing.cc	89.09%	12 Missing ⚠️
.../accessor/grib_accessor_class_bufr_data_element.cc	94.17%	11 Missing ⚠️
...accessor/grib_accessor_class_data_ccsds_packing.cc	88.17%	11 Missing ⚠️
... and 48 more

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #244      +/-   ##
===========================================
- Coverage    87.81%   87.75%   -0.07%     
===========================================
  Files          777      779       +2     
  Lines        62552    62131     -421     
  Branches     11044    11049       +5     
===========================================
- Hits         54933    54520     -413     
+ Misses        7619     7611       -8     

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