SiDetectorElement use double checked locking

Mentioning @oda .

The idea is to use double checked locking to avoid synchronization during the common case of retrieving an already-created cache i.e for readerss accessing const SiDeteElements.

I know that this code might change in the future to try to avoid this kind of lazy initialization with locking.

Anyhow, short term, it seemed to worth the trouble to try something like this for the simple getters in this class as it does not change how easy it is to be removed.