Fix gfortran compilation and some warnings (#410)

# CABLE

Thank you for submitting a pull request to the CABLE Project.

## Description

This PR fixes compilation with gfortran. I also fixes several compiler
warnings (mostly about unused variables) in the MPI master/driver
modules.

## Type of change

Please delete options that are not relevant.

- [X] Bug fix

## Checklist

- [x] The new content is accessible and located in the appropriate
section.
- [x] I have checked that links are valid and point to the intended
content.
- [x] I have checked my code/text and corrected any misspellings


<!-- readthedocs-preview cable start -->
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📚 Documentation preview 📚:
https://cable--410.org.readthedocs.build/en/410/

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src/offline/cable_mpidrv.F90

@@ -24,8 +24,7 @@ PROGRAM mpi_driver
   USE cable_mpicommon
   USE cable_mpimaster
   USE cable_mpiworker
-  USE cable_namelist_util, ONLY: get_namelist_file_name,&
-       CABLE_NAMELIST,arg_not_namelist
+  USE cable_namelist_util, ONLY: get_namelist_file_name
 
   IMPLICIT NONE