bugfix: CAMtr_volume_mixing_ratio read issue

[scrasmussen]

TYPE: bugfix

KEYWORDS: read file, iso_fortran_env

SOURCE: Soren Rasmussen, NCAR

DESCRIPTION OF CHANGES: The iso_fortran_env constant iostat_end has replaced the traditional, but not defined by the Fortran standard, -1 to mean the end-of-file has been reached. This fix reflects that change that is starting to appear in some compilers.

TESTS CONDUCTED: The CLWRF -- clwrf -- CLWRF ALERT! statement had started appearing with the Cray compiler 15.0.1, without any change to the "CAMtr_volume_mixing_ratio.SSP245" input file. This warning message is not showing up anymore.

Checklist

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• Closes issue #xxxx (An issue must exist or be created to be closed. The
  issue describes and documents the problem and general solution, the PR
  describes the technical details of the solution.)
• Tests added (unit tests and/or regression/integration tests)
• Backwards compatible
• Requires new files? If so, how to generate them.
• Fully documented