ReprojectRasterExtent: support single pixel input

[jdries]

#3559

Overview

When a single pixel RasterExtent was reprojected, it often resulted in a GeoAttrsError.
The main cause is warping of square pixels into rectangular.

In the case where we anyway compute a custom cellsize, it seems valid to return a single pixel extent in X or Y direction rather than trying to construct a 0 pixel gridextens, which fails.

Checklist

• ./CHANGELOG.md updated, if necessary. Link to the issue if closed, otherwise the PR.
• Module Hierarchy updated, if necessary
• docs guides update, if necessary
• New user API has useful Scaladoc strings
• Unit tests added for bug-fix or new feature

Closes #3559