fixes #1941

[bena-nasa]

Fixes #1941
This implements the bit shaving as a layout independent algorithm when used.
This is obviously non-zero diff for History diagnostic output when using the nbits option in History unless your application is only being run on a single MPI processes but also obviously has no effect on the module checkpoints and MODEL non-zero diff.

I only marked this as non-zero diff because it will change history diagnostic output. I don't know if that makes zero-diff or non-zero diff, i.e. does the non 0-diff label only apply to restarts.

note per @tclune request I also moved the code to Shared from PFIO, hence why there seems to be a lot of code changes here for a simple algorithmic change.

Also note I had to go against the official MAPL style guide and prepend the c routines in shavemantissa with MAPL_ as there is a copy of this c code in little cfio so if I did not do this, we have name collisions

Description

Related Issue

Motivation and Context

How Has This Been Tested?

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