Add a multi-dimensional PixelFrame

And deprecate Frame2D

gwcs/coordinate_frames.py

@@ -282,6 +282,30 @@ def _world_axis_object_components(self):
         return [(f"{at}{i}" if i != 0 else at, 0, 'value') for i, at in enumerate(self._axes_type)]
 
 
+class PixelFrame(CoordinateFrame):
+    """
+    A coordinate frame describing pixels.
+
+    Parameters
+    ----------
+    naxes : int
+        The number of pixel dimensions described by the frame.
+    name : str, optional
+        The name of this frame.
+    axes_names : list of str, optional
+        The names of the pixel axes.
+    """
+    def __init__(self, naxes, axes_names=None, name=None):
+        super().__init__(
+            naxes,
+            ["PIXEL"]*naxes,
+            axes_order=list(range(naxes)),
+            unit=[u.pix]*naxes,
+            axes_names=axes_names,
+            name=name,
+        )
+
+
 class CelestialFrame(CoordinateFrame):
     """
     Celestial Frame Representation