Handle SimpleITK's lack of thread safety

[bcdarwin]

E.g., setting num_threads to 8 and using loader = simpleitk to read MINC2 files reliably causes a segfault somewhere in HDF5, while num_threads = 1 is fine.

The easiest thing would be to detect multithreaded SimpleITK usage and print a warning that it might lead to problems, although I'd be happy to be able to actually use this configuration.

I'm willing to implement something, just looking to see what people would want.