unify concatenate function

[zoeprieto]

Description

Please include a summary of the change and which issue is fixed if applicable. Please also include relevant motivation and context.

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[coveralls]

Pull Request Test Coverage Report for Build 10290728114

Details

• 0 of 7 (0.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 84.79%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/mgxs.cpp	0	1	0.0%
src/nuclide.cpp	0	1	0.0%
src/thermal.cpp	0	1	0.0%
include/openmc/string_utils.h	0	4	0.0%

Totals
Change from base Build 10288198323:	0.01%
Covered Lines:	49509
Relevant Lines:	58390

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💛 - Coveralls