Add OrderedGroups::MinNonZeroGroup.

Change-Id: If571c2435a7c884b472c33421722208cb2d036ff

include/ceres/ordered_groups.h

@@ -35,6 +35,7 @@
 #include <set>
 #include <vector>
 #include "ceres/internal/port.h"
+#include "glog/logging.h"
 
 namespace ceres {
 
@@ -171,6 +172,14 @@ class OrderedGroups {
     return group_to_elements_.size();
   }
 
+  // The first group with one or more elements. Calling this when
+  // there are no groups with non-zero elements will result in a
+  // crash.
+  int MinNonZeroGroup() const {
+    CHECK_NE(NumGroups(), 0);
+    return group_to_elements_.begin()->first;
+  }
+
   const map<int, set<T> >& group_to_elements() const {
     return group_to_elements_;
   }