Grid geometry lazy load still not thread safe

The used double-checked locking is known to be a non-thread-safe pattern (see e.g. https://www.justsoftwaresolutions.co.uk/threading/multithreading-in-c++0x-part-6-double-checked-locking.html). Making the if check atomic might be bad for performance in case of element map. We could also always make the element map at the cost of memory overhead.