[xmpsdk] Add XMP Initialisation/Cleanup code to all main() programs

- update documentation
- Removed redundant calls to Exiv2::XmpParser::terminate() (legacy of our
  discusssion with @tester0077).
- this fixes #976

samples/write2-test.cpp

@@ -23,7 +23,10 @@ void print(const std::string& file);
 // Main
 int main(int argc, char* const argv[])
 {
-try {
+    Exiv2::XmpParser::initialize();
+    ::atexit(Exiv2::XmpParser::terminate);
+
+    try {
     if (argc != 2) {
         std::cout << "Usage: " << argv[0] << " file\n";
         return 1;