HDF5 support / standard

[bryan-lunt]

Currently any storage of one or more SNOT vectors and the associated structure into HDF5 is done on an ad-hoc basis.

It might be useful to:

• Establish a standard way to store SNOT objects/structure into HDF5.
• Implement some code to do this easily.

(Isn't HDF5 extensible? Even if not, a function/object that can be pointed at an HDF5 subobject would be useful.)