Merge pull request #495 from mantidproject/11429_stitch1dmany_overwrites_output

I tested the changes running this slightly modified usage example, which puts each of the workspaces into a WorkspaceGroup first. Without these changes the two resulting WorkspaceGroups contain the same workspace (the first one is overwritten), after the changes this does not happen, as described in the ticket.

```
trans1 = Load('INTER00013463')
trans2 = Load('INTER00013464')

trans1_wav = CreateTransmissionWorkspaceAuto(trans1)
trans2_wav = CreateTransmissionWorkspaceAuto(trans2)

wsGroup1 = GroupWorkspaces("trans1_wav")
wsGroup2 = GroupWorkspaces("trans2_wav")

stitched_wav1, y = Stitch1DMany("wsGroup1,wsGroup2", params='1, 0.02, 17', UseManualScaleFactor=True, ManualScaleFactor=0.85)
stitched_wav2, y = Stitch1DMany("wsGroup1,wsGroup2", params='1, 0.05, 17', UseManualScaleFactor=True, ManualScaleFactor=3.85)
```

Code/Mantid/Framework/Algorithms/src/Stitch1DMany.cpp

@@ -207,18 +207,20 @@ void Stitch1DMany::exec() {
     // List of workspaces to be grouped
     std::vector<std::string> toGroup;
 
+    const std::string groupName = this->getProperty("OutputWorkspace");
+
     size_t numWSPerGroup = groupWorkspaces[0]->size();
 
     for (size_t i = 0; i < numWSPerGroup; ++i) {
       // List of workspaces to stitch
       std::vector<std::string> toProcess;
       // The name of the resulting workspace
-      std::string outName;
+      std::string outName = groupName;
 
       for (size_t j = 0; j < groupWorkspaces.size(); ++j) {
         const std::string wsName = groupWorkspaces[j]->getItem(i)->name();
         toProcess.push_back(wsName);
-        outName += wsName;
+        outName += "_" + wsName;
       }
 
       IAlgorithm_sptr stitchAlg = createChildAlgorithm("Stitch1DMany");
@@ -245,8 +247,6 @@ void Stitch1DMany::exec() {
                             scaleFactors.end());
     }
 
-    const std::string groupName = this->getProperty("OutputWorkspace");
-
     IAlgorithm_sptr groupAlg = createChildAlgorithm("GroupWorkspaces");
     groupAlg->initialize();
     groupAlg->setAlwaysStoreInADS(true);