A minor usability improvement, I defaulted the feature_vector_type_

to matrix_type.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%404234

dlib/svm/structural_svm_problem.h

@@ -18,7 +18,7 @@ namespace dlib
 
     template <
         typename matrix_type,
-        typename feature_vector_type_ 
+        typename feature_vector_type_ = matrix_type
         >
     class structural_svm_problem : public oca_problem<matrix_type> 
     {

dlib/svm/structural_svm_problem_abstract.h

@@ -14,7 +14,7 @@ namespace dlib
 
     template <
         typename matrix_type,
-        typename feature_vector_type_ 
+        typename feature_vector_type_ = matrix_type
         >
     class structural_svm_problem : public oca_problem<matrix_type> 
     {