Fix ResampledLowLevelWCS pixel space conversions

Co-authored-by: Dan Ryan <ryand5@tcd.ie>

Author: Cadair

ndcube/wcs/wrappers/resampled_wcs.py

@@ -1,3 +1,4 @@
+
 import numpy as np
 
 from astropy.wcs.wcsapi.wrappers.base import BaseWCSWrapper
@@ -42,13 +43,13 @@ def _top_to_underlying_pixels(self, top_pixels):
         # Convert user-facing pixel indices to the pixel grid of underlying WCS.
         factor = self._pad_dims(self._factor, top_pixels.ndim)
         offset = self._pad_dims(self._offset, top_pixels.ndim)
-        return (top_pixels + factor/2) * factor + offset
+        return (top_pixels * factor) + 1 + offset
 
     def _underlying_to_top_pixels(self, underlying_pixels):
         # Convert pixel indices of underlying pixel grid to user-facing grid.
         factor = self._pad_dims(self._factor, underlying_pixels.ndim)
         offset = self._pad_dims(self._offset, underlying_pixels.ndim)
-        return (underlying_pixels - offset) / factor
+        return (underlying_pixels - 1 - offset) / factor
 
     def _pad_dims(self, arr, ndim):
         # Pad array with trailing degenerate dimensions.