Add Tally.from_hdf5 method.

[pshriwise]

Description

In #2671 a method Statepoint._populate_tally was added that wasn't quite appropriate for the purpose of that PR, but it might make for a nice Tally.from_hdf5 method that allows us to compartmentalize some of the Tally object creation that occurs in the Statepoint object currently.

openmc/openmc/statepoint.py

Line 429 in 2fb7e3d

def _populate_tally(self, tally):

Alternatives

Leave well enough alone

Compatibility

This would be an internal refactor and only expose new functionality on the Tally class.