Add parallel `load_npy` functions to Context and DenseLocalArray

Add `load_npy` functions for loading in a `.npy` file in parallel.  This code is very similar to the recent `load_hdf5` code.

I believe it will also be fairly simple to load these files in as memory maps.

This is based on and subsumes PR #205: loading an .hdf5 file in parallel.

I have also redone the IO tests to use setUp and tearDown methods for the file management as @cowlicks suggested.