Address ignored fortitude rules for Fortran

Programming language: Fortran

See [fortitude.toml](/MetOffice/shumlib/blob/main/fortitude.toml)

[Rules](https://fortitude.readthedocs.io/en/stable/rules/) that are currently ignored:

- [x] `C002` interface-implicit-typing. #22
- [ ] `C003` implicit-external-procedures
- [x] `C061` missing-intent. #22
- [ ] `C071` assumed-size
- [ ] `C072` assumed-size-character-intent
- [x] `C081` initialisation-in-declaration. #22
- [x] `C121` use-all. #22
- [ ] `C141` missing-exit-or-cycle-label
- [ ] `E001` syntax-error
- [x] `MOD011` old-style-array-literal. #22
- [x] `MOD021` deprecated-relational-operator. #22
- [x] `S061` unnamed-end-statement. #22

Start with fortitude.toml and the Fortitude rules documentation, then inspect the unchecked rules: C003, C071, C072, C141, and E001. Determine the changes needed across the Fortran sources to satisfy them, run Fortitude to verify the rules no longer need to be ignored, and update the checklist when all remaining rules pass.

Written by the indexing model from the issue text.