statistics-mtg2 configuration/options cleanup

[tweska]

• get rid of some unused methods
• make all variables const
• simplify the "time utils", and get rid of the unused/duplicate methods
• rename some methods/variables to follow the usual convention

🌈🌦️📖🚧 Documentation 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/dev-section/multio/pull-requests/PR-168

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.20792% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.60%. Comparing base (0d96979) to head (525a76b).

Files with missing lines	Patch %	Lines
...io/action/statistics-mtg2/cfg/StatisticsOptions.cc	78.02%	20 Missing ⚠️
...ion/statistics-mtg2/cfg/StatisticsConfiguration.cc	80.41%	19 Missing ⚠️
...s-mtg2/operations/OperationWithDeaccumulatedData.h	50.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #168      +/-   ##
===========================================
- Coverage    56.61%   56.60%   -0.02%     
===========================================
  Files          318      318              
  Lines        20650    20500     -150     
  Branches      1568     1569       +1     
===========================================
- Hits         11692    11603      -89     
+ Misses        8958     8897      -61     

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