Support creating strand objects from HDF5

[wtbarnes]

Currently, when instantiating a Strand, it has to be backed by a full HYDRAD directory. We should also support creating strands from HDF5 files that are written by the to_hdf5 method.

This could be done fairly easily by adapting the property template function to optionally pull from an HDF5 file.

An even better method would be to completely abstract away the backing data model from the Strand object so that the data format could be more generic.