Disable console notification by default within the library

As a library, the default shouldn't be to write anything on console.
`cover` and `fastcover` have a `g_displayLevel` variable to control this behavior.
It's now set to 0 (no display) by default.
Setting notification to a higher level should be an explicit operation by a console application.

lib/dictBuilder/cover.c

@@ -47,7 +47,7 @@
 *  Console display
 ***************************************/
 #ifndef LOCALDISPLAYLEVEL
-static int g_displayLevel = 2;
+static int g_displayLevel = 0;
 #endif
 #undef  DISPLAY
 #define DISPLAY(...)                                                           \
@@ -735,7 +735,7 @@ ZDICTLIB_API size_t ZDICT_trainFromBuffer_cover(
   COVER_map_t activeDmers;
   parameters.splitPoint = 1.0;
   /* Initialize global data */
-  g_displayLevel = parameters.zParams.notificationLevel;
+  g_displayLevel = (int)parameters.zParams.notificationLevel;
   /* Checks */
   if (!COVER_checkParameters(parameters, dictBufferCapacity)) {
     DISPLAYLEVEL(1, "Cover parameters incorrect\n");