Support the HyperSpy import/export format

[cmeyer]

See HSpy - HyperSpy's HDF5 Specification.

For the simple case, we can read/write the HSpy format without importing the hyperspy library. This could go into this project (nionswift-io) since it would not add a new dependency.

We should also revisit the idea of using the whole set of HyperSpy import/export formats. Doing this would instantly add numerous other file formats. One issue to resolve before heading in this direction is the huge footprint of HyperSpy since it imports so many libraries; and the fact that it is not easily installed into an environment using the default conda channel and instead requires the conda-forge channel. This would have to go into the nionswift-hyperspy project (stale, at the moment) since it would require importing the hyperspy library.