MAPL3: Move to implicit none type external

[mathomp4]

Types of change(s)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Trivial change (affects only documentation or cleanup)
• Refactor (no functional changes, no api changes)

Checklist

• Tested this change with a run of GEOSgcm
• Ran the Unit Tests (make tests)

Description

This is the MAPL3 version of #3002 . We don't expect to ever build MAPL3 with nvfortran so the issues with implicit none (type, external) are moot.

This PR updates MAPL3 to use implicit none (type, external) in most of MAPL and implicit none (type) in places with:

• use mpi
• use of netcdf.inc
• One place with OpenMP (I think)

It seems to pass a local build. Now to see what CI thinks.

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