also strip 'filename' postfix from field names
improve escapeShellArg() and its docstring
remove unused ArgumentConverter members
cleanup using finalizeCommand (no context manager)
reuse input image files if possible
restore functionality of retainTemporaryFiles

(This is no longer really necessary, since the files are retained as
long as the module lives and the inputs do not change, but it may still
be useful if one exits MeVisLab and wants to debug externally.)