Modify Results.from_hdf5 to support for files w/o depletion time
Modify Results.from_hdf5 to support for files w/o depletion time

Needed because ./tests/regression_tests/deplete/test_reference.h5
does not contain this dataset. Now, a check is performed
to see if "depletion file" is present in the root of the hdf5 file.
Otherwise, the proc_time is set to the array of nans

walshjon/openmc,liangjg/openmc,walshjon/openmc,paulromano/openmc,liangjg/openmc,mit-crpg/openmc,amandalund/openmc,amandalund/openmc,paulromano/openmc,liangjg/openmc,shikhar413/openmc,walshjon/openmc,walshjon/openmc,shikhar413/openmc,mit-crpg/openmc,paulromano/openmc,amandalund/openmc,mit-crpg/openmc,liangjg/openmc,shikhar413/openmc,smharper/openmc,smharper/openmc,paulromano/openmc,smharper/openmc,amandalund/openmc,shikhar413/openmc,smharper/openmc,mit-crpg/openmc