Add attr function for xercesc use, so that char arrays can be used to get xml...

Add attr function for xercesc use, so that char arrays can be used to get xml elements (e.g., necessary to use ddsim w/ xercesc)

DDCore/include/XML/XMLElements.h

@@ -693,6 +693,12 @@ namespace DD4hep {
       template <class T> T attr(const XmlChar* tag) const {
         return m_element.attr<T>(tag);
       }
+#ifndef __TIXML__
+      /// Access typed attribute value by it's name
+      template <class T> T attr(const char* name) const {
+        return this->attr<T>(Strng_t(name));
+      }
+#endif
       /// Access attribute name (throws exception if not present)
       const XmlChar* attr_name(const Attribute attr) const {
         return m_element.attr_name(attr);