cleanup of mpp::get_peset function

[bensonr]

Description
Minor cleanup and performance enhancements for the mpp::get_peset function

Fixes #1273

How Has This Been Tested?
FMS CI was tested on an AMD-based dev box and Gaea. Additional testing was performed with a test code that calls mpp_broadcast with an optional undefined pelist which will ensure code coverage for the modified sections of get_peset.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes

[thomas-robinson]

@rem1776 make sure this is included in 2023.03