File format for decay data

[paulromano]

To use this information effectively in a simulation, we'll want to save the decay data in an HDF5 format. We'll need to decide if we want to save it as part of the existing XS datasets, leading toward single-source data libraries, or as separate decay data libraries with their own specification.

[paulromano]

We already have decay photon data that is stored in depletion chain files thanks to openmc-dev/openmc#2234, so the easiest path forward is to simply have the C++ layer read data directly from the chain file (determined from OPENMC_CHAIN_FILE) rather than trying to put it in an HDF5 format.

[paulromano]

Closed with openmc-dev/openmc#3235