Detect parallel h5py

Here I add a function `import_parallel_h5py` to `testing.py` that tries to import a parallel-enabled h5py, and if one isn't available, it raises `unittest.SkipTest`.  This replaces most of our uses of `import_or_skip` (but not all) and calls `import_or_skip` under the hood.

Unfortunately, @kwmsmith, it seems to make our paralleltests hang... can you see why Kurt?

Not ready for merge.