rewrite resample to use CoordinateGenerator

RegularGrid.resample now uses CoordinateGenerator rather than building
arrays with numpy.meshgrid

This may make future efficiency improvements easier as they can be
focused on CoordinateGenerator

karta/raster/grid.py

@@ -756,21 +756,23 @@ def resample(self, dx, dy, method='nearest'):
                              "(got {0}, {1})".format(dx, dy))
 
         xmin, xmax, ymin, ymax = self.get_extent()
+        ny = int((ymax - ymin) // (dy / self.resolution[1])) + 1
+        nx = int((xmax - xmin) // (dx / self.resolution[0])) + 1
+
+        t = self._transform
+        tnew = (xmin-0.5*dx-0.5*t[4], ymin-0.5*dy-0.5*t[5], dx, dy, t[4], t[5])
+        cg = CoordinateGenerator(tnew, (ny, nx), self.crs, self.crs)
+        X, Y = cg[:,:]
+
         if method == 'nearest':
-            X, Y = np.meshgrid(np.arange(xmin, xmax, dx),
-                               np.arange(ymin, ymax, dy))
             values = self.sample_nearest(X, Y)
         elif method == 'linear':
-            X, Y = np.meshgrid(np.arange(xmin, xmax, dx),
-                               np.arange(ymin, ymax, dy))
             values = self.sample_bilinear(X, Y)
         else:
             raise NotImplementedError('method "{0}" unavailable'.format(method))
 
         if values.ndim == 3:
             values = values.transpose(1, 2, 0)
-        t = self._transform
-        tnew = (xmin-0.5*dx-0.5*t[4], ymin-0.5*dy-0.5*t[5], dx, dy, t[4], t[5])
         return RegularGrid(tnew, values=values, crs=self.crs,
                            nodata_value=self.nodata)