Fix Compiler Warning. refs #3868 & #4297

Code/Mantid/Framework/DataObjects/src/PeaksWorkspace.cpp

@@ -238,7 +238,7 @@ namespace DataObjects
   {
 
     //Number of Peaks
-    const int np(peaks.size());
+    const size_t np(peaks.size());
 
     // Column vectors for peaks table
     std::vector<int> detectorID(np);
@@ -466,4 +466,4 @@ namespace Mantid
   } // namespace Kernel
 } // namespace Mantid
 
-///\endcond TEMPLATE
\ No newline at end of file
+///\endcond TEMPLATE