Fix a couple of bugs that disabled antialiasing in regrid and resample.

In _antialias_filter_image(), a test that was supposed to skip the antialiasing step if neither axis needed antialiasing, was inverted. Instead it only skipped the antialiasing step if both axes needed antialiasing.

In Image.regrid() both places that consulted and updated the current spatial-frequency limits operated on self instead of the image (which differs from self when copy is True).

Added an Image.copy() function which first calls the DataArray copy function to copy the generic parts of the object, then copies attributes that are specific to Images. Currently these are self._spflims and self._has_wcs.