Generalized indices: Refactor group_update for improved performance

[J-Lentz]

Description
Refactor the packing and unpacking routines of mpp_group_update for improved performance with generalized indices. The outer k-loop versions of the packing and unpacking routines have been removed.

How Has This Been Tested?
Builds with ifx on C5. Relevant unit tests fail.

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

[uramirez8707]

I tested this branch with AM5 to compare performance, but I am getting a seg fault error

forrtl: severe (174): SIGSEGV, segmentation fault occurred
/gpfs/f5/gfdl_f/scratch/Uriel.Ramirez/volatile/am5/am5f12e3r0_2026.02-alpha3-candidate/c96L65_am5f12e3r0_amip/ncrc5.intel25-prod-openmp/stdout/run/c96L65_am5f12e3r0_amip_1x0m8d_576x2a.o135778381

The error appears to be consistent.
1 thread jobs fail as well:
/gpfs/f5/gfdl_f/scratch/Uriel.Ramirez/volatile/am5/am5f12e3r0_2026.02-alpha3-candidate/c96L65_am5f12e3r0_amip/ncrc5.intel25-prod-openmp/stdout/run/c96L65_am5f12e3r0_amip_1x0m8d_576x1a.o135778386

c384 failed as well:
/gpfs/f5/gfdl_f/scratch/Uriel.Ramirez/volatile/am5/am5f12e3r0_2026.02-alpha3-candidate/c384L65_am5f12e3r0_amip/ncrc5.intel25-prod-openmp/stdout/run/c384L65_am5f12e3r0_amip_1x0m8d_1728x4a.o135778376