Conversion of expressions and arguments to C++

[joobog]

https://jira.ecmwf.int/browse/ECC-1982

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.99549% with 195 lines in your changes missing coverage. Please review.

Project coverage is 87.75%. Comparing base (dd6bdb9) to head (7e8a006).

Files with missing lines	Patch %	Lines
src/expression/grib_expression_class_is_integer.cc	56.86%	22 Missing ⚠️
src/expression/grib_expression_class_is_in_dict.cc	76.38%	17 Missing ⚠️
src/expression/grib_expression.cc	20.00%	16 Missing ⚠️
src/expression/grib_expression_class_is_in_list.cc	81.17%	16 Missing ⚠️
src/expression/grib_expression_class_logical_or.cc	82.45%	10 Missing ⚠️
src/expression/grib_expression_class_accessor.cc	86.88%	8 Missing ⚠️
src/expression/grib_expression_class_length.cc	81.81%	8 Missing ⚠️
src/expression/grib_expression_class_sub_string.cc	73.33%	8 Missing ⚠️
src/expression/grib_expression_class_unop.cc	74.19%	8 Missing ⚠️
src/geo_nearest/grib_nearest_class_reduced.cc	63.63%	8 Missing ⚠️
... and 30 more

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #271      +/-   ##
===========================================
- Coverage    87.79%   87.75%   -0.04%     
===========================================
  Files          809      827      +18     
  Lines        62140    62050      -90     
  Branches     11039    11018      -21     
===========================================
- Hits         54558    54455     -103     
- Misses        7582     7595      +13     

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