Parse remaining fields for RMC sentence

[clemarescx]

This PR solves issue #60 and adds parsing of the remaining fields of RMC sentences:

• 10 + 11: magnetic variation in degrees + direction of variation
• 12: FAA Mode (NMEA v2.3 and above)
• 13: Navigational status (NMEA v4.1 and above)

I added a couple more basic tests to check for sentences with 11, 12, or 13 fields, as well as some other tests for missing fields.

Magnetic variation

Since magnetic variation is a field combination (degrees + E/W) in a couple of other sentences (HDG and RMA), I added a combinator in utils for it that can be reused. However I'm not sure whether my implementation is correct, I followed the explanations from Magnetic Declination 3.2 - Non-adjustable compasses, but please double-check that both implementation and test are correct.

Navigational status

Following the discussion on Discord, I found a specification of RMC by Orolia, where the values used for the nav status field are different, and conflicting with the ones mentioned by GPSD's RMC description. It was decided that we should stick to GPSD compliance for the current implementation and parse the field into an RmcNavigationStatus enum value instead of a char.

[elpiel]

Thank you for working on this issue with RMC sentences. I will make sure to review the PR in the coming days.

[elpiel]

LGTM!
The only change is the double (f64) usage in one of the parsers.

I've opened a separate issue to investigate whether f64 accuracy is needed and if so, to find option that will allow the user to choose between f64 & f32: #67

[codecov]

Codecov Report

Base: 79.24% // Head: 79.41% // Increases project coverage by +0.16% 🎉

Coverage data is based on head (9c36d7a) compared to base (b73418a).
Patch coverage: 81.48% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
+ Coverage   79.24%   79.41%   +0.16%     
==========================================
  Files          22       22              
  Lines         824      850      +26     
==========================================
+ Hits          653      675      +22     
- Misses        171      175       +4     

Impacted Files	Coverage Δ
src/sentences/rmc.rs	82.22% <70.58%> (-7.07%)	⬇️
src/sentences/utils.rs	100.00% <100.00%> (ø)
src/sentences/faa_mode.rs	60.86% <0.00%> (+2.17%)	⬆️

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[elpiel]

Thank you for working on this issue! 🎉