Add FGPositioned static methods: isAirportType(), isRunwayType(), isNavaidType()

For consistency, define these three static methods in FGPositioned.
FGPositioned::isAirportType() is the same as FGAirport::isAirportType()
(piece of code moved from airport.cxx to positioned.cxx, and
FGAirport::isAirportType() now calls FGPositioned::isAirportType()).

- FGPositioned::isAirportType() returns true for AIRPORT, HELIPORT,
  SEAPORT;
- FGPositioned::isRunwayType() returns true for RUNWAY;
- FGPositioned::isNavaidType() returns true for NDB, VOR, ILS, LOC, GS,
  DME, TACAN.

src/Airports/airport.cxx

@@ -51,6 +51,7 @@
 #include <ATC/CommStation.hxx>
 #include <Navaids/NavDataCache.hxx>
 #include <Navaids/navrecord.hxx>
+#include <Navaids/positioned.hxx>
 #include <Airports/groundnetwork.hxx>
 #include <Airports/xmlloader.hxx>
 
@@ -106,13 +107,10 @@ bool FGAirport::isHeliport() const
   return type() == HELIPORT;
 }
 
+// Static method
 bool FGAirport::isAirportType(FGPositioned* pos)
 {
-    if (!pos) {
-        return false;
-    }
-    
-    return (pos->type() >= AIRPORT) && (pos->type() <= SEAPORT);
+  return FGPositioned::isAirportType(pos);
 }
 
 //------------------------------------------------------------------------------

src/Airports/airport.hxx

@@ -74,6 +74,8 @@ class FGAirport : public FGPositioned
         return mIsClosed;
     }
 
+    // True for the following types: AIRPORT, HELIPORT, SEAPORT.
+    // False for other types, as well as if pos == nullptr.
     static bool isAirportType(FGPositioned* pos);
 
     virtual const std::string& name() const

src/Navaids/positioned.cxx

@@ -85,6 +85,43 @@ FGPositioned::~FGPositioned()
 {
 }
 
+// Static method
+bool FGPositioned::isAirportType(FGPositioned* pos)
+{
+    if (!pos) {
+      return false;
+    }
+
+    return (pos->type() >= AIRPORT) && (pos->type() <= SEAPORT);
+}
+
+// Static method
+bool FGPositioned::isRunwayType(FGPositioned* pos)
+{
+  return (pos && pos->type() == RUNWAY);
+}
+
+// Static method
+bool FGPositioned::isNavaidType(FGPositioned* pos)
+{
+  if (!pos) {
+    return false;
+  }
+
+  switch (pos->type()) {
+  case NDB:
+  case VOR:
+  case ILS:
+  case LOC:
+  case GS:
+  case DME:
+  case TACAN:
+    return true;
+  default:
+    return false;
+  }
+}
+
 FGPositionedRef
 FGPositioned::createUserWaypoint(const std::string& aIdent, const SGGeod& aPos)
 {

src/Navaids/positioned.hxx

@@ -98,6 +98,16 @@ public:
   Type type() const
   { return mType; }
 
+  // True for the following types: AIRPORT, HELIPORT, SEAPORT.
+  // False for other types, as well as if pos == nullptr.
+  static bool isAirportType(FGPositioned* pos);
+  // True for the following type: RUNWAY.
+  // False for other types, as well as if pos == nullptr.
+  static bool isRunwayType(FGPositioned* pos);
+  // True for the following types: NDB, VOR, ILS, LOC, GS, DME, TACAN.
+  // False for other types, as well as if pos == nullptr.
+  static bool isNavaidType(FGPositioned* pos);
+
   const char* typeString() const
   { return nameForType(mType); }