Adding KM visibility

Merge pull request #26 from falcon71/master

Visibility in km

Visibility "NDV" + wiser parsing continuation on errors

• The Visibility object includes a NDV flag (No Directional Variation) (Issue raised by @filerun #14)
• Better parsing continuation on errors in non-strict mode (Issue raised by @gpdawson #13)

Value conversion on the fly

• Access to converted value with $value->getConvertedValue($unit)
  Supported values: speed, distance and pressure. Details in README. Developed by @JohnieWalker

Wiser decoding continuation on errors

This change only concerns non-strict decoding mode.

• In case of error while decoding the METAR, the next chunk to use is now determined to maximise the odds of proper parsing continuation.

More optional fields + choice between strict or not-strict decoding

• Temperature: chunk is now optionnal
• Visibility: chunk can contain "no information"
• PresentWeather: chunk can contain "no information"
• Allow multiple errors to be trigerred while decoding
• Parsing can be done in "strict" or "not strict" mode (in strict mode, parsing is interrupted on first error)
• Parsing mode can be set globally

Fixing bugs detected during first batch on real data

• MetarStatus: allow any 3-letters code for status (only AUTO and NIL where allowed before)
• RunwayVisualRange: allow visual range intervals (e.g. R01/3000V4000FT)
• PresentWeather: BR code is recognised (there was a typo)
• VerticalVisibility: VV code is handled as any other cloud code
• RecentWeather: use exactly the same pattern as PresentWeather ([0->1] weather characteristics + [0->2] weather types)
• Windshear: handle windshear for several runways

Massive tests to be continued

ICAO compliance + US specificities compliance

Taking into account feedbacks from field expert
Still needs to be tested with massive amount of real data

First version with complete perimeter coverage (ICAO standard)

Need field expert feedback