Make parallel_sextractor() work on a copy of the input image
Update docstring of parallel_sextractor()
Copy each temporary output image to the actual output path
Register clean-up functions to remove the temporary output images
Make sure temporary output images are user-writable
Remove temporary output images as they become no longer necessary
Do not modify input FITS files when SExtractor is run

FITSeeingImage.__init__() writes to the header of the FITS file both
the path to the on-disk SExtractor catalog and the MD5 hash of the
configuration files — provided, of course, that it is user-writable.
The problem with this approach is that it means that the 'seeing'
command modifies the input FITS files. This is arguably not what the
user might expect, since it defeats most of the purpose of having to
specify an output directory.

Fix this by working on a copy of the input images. In this manner,
FITSeeingImage.__init__() modifies the header of the file that it
receives as argument, but the original images remain untouched.

This merges branch 'dont-modify-input-seeing-images'