DOC, TEST: Add test and doc to warp and frame packer

Adding more documentation and tests for **Warper** and **ProjectorFramePacker**. 

(In retrospect, it would have been nice to have the class names match the file names.)

I added .rst files for each class and verified each gets added to the documentation index after building, but I didn't know the best way to further integrate into the documentation so I'm leaving that to someone else.

Also added a sample warp file which is referenced by **test_projections.py**.