Refine message displaying. Now, we have :

Refine message displaying. Now, we have :
 * Warning  : Warns the user when some oddity occured, and we used an heuristics
 *            to bypass the trouble.
 *            e.g.  : SamplesPerPixel missing, we assume it's a grey level image
 *            e.g   : Pixel Representation missing : we assume 'unsigned'
 *            (we hope our assumption is OK, but user may disagree.)
 * Log      : we display messages when anything is not Dicom Kosher
 *            e.g. : non even length field
 *            e.g  : file is declared as Explicit VR, but a DataElement
 *                 is Implicit
 *            e.g  : a file holds an illegal group (0x0005, ...)
 * Debug : We help developper to trace, at a glance, the execution.
           (before refining with a debugging tool)
 * Setting ON Debug leads to set ON Warning (but not Log)
 * Setting ON Log   leads to set ON Warning (but not Debug)