Rethinking the "SExtractor workdir exists" warning

[mtewes]

Maybe this should simply be debug-level, not warning-level, and the issue is solved.
A warning should occur only when a file is overwritten.
But this subject is related to the other issues (how much freedom the user has to specify CATALOG_NAME), and the best is probably to fix the other issues first, before making the easiest change.

[kuntzer]

Maybe we should make the user choose how it should handle existing files. By default SExtractor overwrites silently, so we should use the same choice by default.

[mtewes]

Woops, a few months have passed without any activity on sewpy. As this warning is so annoying, I'll now at least comment it out...