Add from_hdf5() in `openmc.data.IncidentPhoton`
Created by: liangjg
This PR, separated from #1203 , is a refactoring of photon-related Python API, by adding
to_hdf5 methods for better photon data processing.
One thing to note is that
log-log interpolation scheme is mostly used in photon tablulated data. Zero values are changed to small non-zero data to enable
log operations. Zero values come from ENDF tables which uses
lin-lin interpolation for the first points in the zero xs range.
xs[xs == 0.0] = np.exp(-500.0)