Ogimet improvements and SYNOP parser

[bczernecki]

This PR addresses quite numerous changes that were hanging for some time:

• Including SYNOP parser/decoder for meteorological data - mostly based on Ogimet service
• refactor of meteo_ogimet()
• adding telemetrical/datastore resources for meteo_imgw() wrapper
• minor fixes in hydro_imgw
• computing relative humidity based on air temperature and dew_point temperature to adjust output to rendered in the ogimet's tables
• updates in documentation

[codecov]

Codecov Report

❌ Patch coverage is 91.38943% with 132 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.35%. Comparing base (a9deb55) to head (8af6b91).

Files with missing lines	Patch %	Lines
R/parser.R	92.69%	88 Missing ⚠️
R/meteo_ogimet_synop.R	88.65%	22 Missing ⚠️
R/meteo_ogimet.R	76.81%	16 Missing ⚠️
R/meteo_imgw.R	73.91%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
+ Coverage   87.81%   91.35%   +3.53%     
==========================================
  Files          33       36       +3     
  Lines        1839     3331    +1492     
==========================================
+ Hits         1615     3043    +1428     
- Misses        224      288      +64     

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