Update Standalone MAPL Python to work with Python 2 and 3

[mathomp4]

Description

This PR updates 3 of the Python scripts in MAPL to work with Python 2 and 3 using 2to3 as well as hints from:

http://python-future.org/compatible_idioms.html

I was able to test both:

• base/mapl_tree.py
• base/mapl_vlist.py

and tree was zero diff, and vlist was...actually more accurate in Python 3. But the Python 2 output was not changed.

As for Apps/MAPL_GridCompSpecs_ACG.py, I wasn't sure how to test it, but the change 2to3 made was slight and I can't see how it could fail.

NOTE: This PR does not change the scripts in Python/MAPL as I'm not too sure how to test those.

Related Issue

Motivation and Context

Transition from Python 2 to 3 is needed soon.

How Has This Been Tested?

I ran each of the scripts to make sure it worked (save the ACG one which I can't imagine won't)

I also ran a 1-day c24 run of GEOS with MAPL develop versus this branch. It built and was zero-diff.

Types of changes

• 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)

Checklist:

• I have tested this change with a run of GEOSgcm (if non-trivial)
• I have added one of the required labels (0 diff, 0 diff trivial, 0 diff structural, non 0-diff)
• I have updated the CHANGELOG.md accordingly following the style of Keep a Changelog