Cardinal/OpenMC is generating compiler warnings

[cticenhour]

Bug Description

Current FENIX debug testing builds with -Werror enabled to check for compiler warnings. With the merge of #53, this check will fail. See relevant PR result from CIVET here: https://civet.inl.gov/job/2387476/

EDIT: This occurs for scheduled Mac testing too: https://civet.inl.gov/job/2423456/

Steps to Reproduce

Build FENIX in debug mode with CPPFLAGS += -Werror

Impact

Doesn't impact code functionality, but impinges on FENIX code standards and build cleanliness.