Cannot parse METARs with both COR and AUTO modifiers #15

Description:

ICAO-format METARs commonly have both correction (COR) and automation (AUTO) modifiers together, but SwiftMETAR's Observer enum can only represent one state:

public enum Observer: String, Codable {
    case human = ""
    case automated = "AUTO"
    case corrected = "COR"
}

Current Behavior:

// ✅ Works
try await METAR.from(string: "EGLL 172050Z AUTO 08005KT 9999 NCD 03/M03 Q1016")
// observer = .automated

// ✅ Works
try await METAR.from(string: "EGLL 172050Z COR 08005KT 9999 NCD 03/M03 Q1016")
// observer = .corrected

// ❌ Fails with badFormat error
try await METAR.from(string: "EGLL 172050Z COR AUTO 08005KT 9999 NCD 03/M03 Q1016")

Expected Behavior:

Should parse successfully and preserve both pieces of information. The actual weather data (wind, temperature, QNH, etc.) is valid - the parser shouldn't fail entirely just because both modifiers are present.

Real-World Impact:

Many European automated weather stations issue corrected reports with this format. Currently, all such METARs fail to parse even though the weather data itself is perfectly valid.

Suggested Approaches:

1. Separate boolean properties (simplest):

public var isAutomated: Bool
public var isCorrected: Bool

2. OptionSet (more flexible):

public struct ObserverFlags: OptionSet {
    public let rawValue: Int
    public static let automated = ObserverFlags(rawValue: 1 << 0)
    public static let corrected = ObserverFlags(rawValue: 1 << 1)
}
public var observerFlags: ObserverFlags

3. Graceful degradation (quickest fix):
   Parse either modifier and succeed, even if we can only store one. Parsing COR AUTO and only capturing .automated is better than failing completely.

References:

• ICAO Annex 3 / Doc 8896 allows multiple modifiers
• Example: Many UK automated stations (EGLL, EGKK, EGGD) regularly issue COR AUTO reports