HDF5 output support

[munkm]

This issue is to capture a discussion from the cyclus hackathon.

Right now we support HDF5 and sqlite outputs for cyclus. Is HDF5 favored as an output format for our users, or is sqlite the preferred output format? It's possible we could relieve some maintenance burden for our community if we choose to sunset HDF5 support in the future. Is there a benefit that this output format brings that sqlite doesn't?

[gonuke]

During the original development and first wave of Cyclus work, there was a strong affinity for maintaining the HDF5 backend, at least among some developers, because of performance concerns for SQL if/when simulations became large.