BUG: Fix bug in xpointers when writer is used multiple times.

XPointers were generated incorrectly when the same XdmfWriter was
used to write files multiple times. This was caused by not resetting
xPathCount when after the file was written.

core/XdmfWriter.cpp

@@ -68,7 +68,10 @@ public:
   void
   closeFile()
   {
+    // remove xpath info as it will not be valid for the new file
     mXPath.clear();
+    mXPathCount = 0;
+    mXPathString = "";
 
     std::ofstream fileStream;
     if(!mStream) {