Changed requires clause slightly to increase usability.

dlib/image_processing/scan_fhog_pyramid.h

@@ -928,7 +928,7 @@ namespace dlib
     )
     {
         // make sure requires clause is not broken
-        DLIB_ASSERT(thresh > 0 ,
+        DLIB_ASSERT(thresh >= 0 ,
             "\t object_detector threshold_filter_singular_values()"
             << "\n\t Invalid inputs were given to this function."
             << "\n\t thresh: " << thresh 

dlib/image_processing/scan_fhog_pyramid_abstract.h

@@ -66,7 +66,7 @@ namespace dlib
     );
     /*!
         requires
-            - thresh > 0
+            - thresh >= 0
             - weight_index < detector.num_detectors()
             - detector.get_w(weight_index).size() >= detector.get_scanner().get_num_dimensions()
               (i.e. the detector must have been populated with a HOG filter)