implement Flip feature

lib/flip.dart

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+library turf_flip;
+
+export 'package:geotypes/geotypes.dart';
+export 'src/flip.dart';

lib/src/flip.dart

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+import 'package:turf/turf.dart';
+import 'package:turf/meta.dart';
+
+// Takes a set of input Coordinate features and flips their Coordinates from Position(x,y) to Position(y, x) 
+
+// Returns a GeoJSON FeatureCollection with the flipped Coordinates
+
+Feature<GeometryType> flip(Feature<GeometryType>? geojson,
+{Map<String,dynamic>? properties, }) {
+  if (geojson == null) {
+    throw ArgumentError('geojson cannot be null');
+  }  
+
+  coordEach(
+    geojson,
+    (Position? currentCoord, int? coordIndex, int? featureIndex, int? multiFeatureIndex, int? geometryIndex) {
+      currentCoord = Position.named(lat: currentCoord!.lng, lng: currentCoord.lat);
+    }
+  );
+
+  return geojson;
+}

test/components/flip_test.dart

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+import 'package:test/test.dart';
+import 'package:turf/flip.dart';
+
+void main() {
+  group('flip tests', () {
+    test('flipping individual coordinates - point', () {
+      var point = Feature(
+        geometry: Point(
+          coordinates: Position.named(lat: -75, lng: 39)
+        ),
+      );
+
+      expect(flip(point).geometry?.coordinates, equals(Position(39,-75)));
+    });
+
+    test('flipping a group of coordinates - polygon', () {
+      var polygon = Feature(
+        geometry: Polygon(
+          coordinates: [
+            [
+            Position.named(lat: -75, lng: 39),
+            Position.named(lat: -75, lng: 38),
+            Position.named(lat: -74, lng: 38),
+            Position.named(lat: -74, lng: 39),
+            Position.named(lat: -75, lng: 39),
+            ],
+          ], 
+        ),
+      );
+
+      var expPolygon = [
+        [
+          Position(39,-75),
+          Position(38, -75),
+          Position(38, -74),
+          Position(39, -74),
+          Position(39, -75)
+        ]
+      ];
+      expect(flip(polygon).geometry?.coordinates, equals(expPolygon));
+
+    });
+  });
+}