Use string_t in micm and tuvx f90 musica fortran files

[K20shores]

Tuvx and micm are not using the string type that was created for the mapping. Use the string type so that we are consistently passing strings through our Fortran-C interface

Related
#166

Acceptance criteria

• MICM constructor c api uses the string_t type
• TUVX consturctor c api uses the string_t type

[github-actions]

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