Created by: nelsonag

This PR resolves the issue identified in #816 (closed).

It also:

• Fixes issues on my machine with test_mg_convert and test_mgxs_library_ce_to_mg in which they failed to run without closing the statepoint and summary files. (these used to be in there?)
• Adds more tally types to test_mg_tallies to cover the branch of code in question
• Fixes an issue I just ran in to on my machine wherein test_mgxs_library_mesh fails. This might be the exact issue causing pains for @samuelshaner in #811, give the fix a look