Draft HDF5 converter

[michael-petersen]

Based on a user request, I've been thinking about how to best implement HDF5 structure for file outputs. This is a draft implementation of an HDF5 phase-space system.

Still to be done:

• Add an HDF5 reader for easier interface
• Verify that this can be used externally with static methods
• Decide if the group strategy for headers makes the most sense