Add Perlmutter CPU as balsam site

[tylern4]

I've been working with @cms21 on the DOE HPC Workflow training and wanted to add a Perlmutter CPU site for our users. This also makes some updates to the queue policies on Perlmutter from our docs. https://docs.nersc.gov/jobs/policy/#perlmutter-gpu

[cms21]

We are going to go with this approach for now since it's implemented and working, but we may want to revisit it in the future. Another possible approach is to pass a parameter in the Batch Job instance that differentiates between gpu/cpu.

[codecov-commenter]

Codecov Report

Patch coverage: 83.33% and project coverage change: +0.01 🎉

Comparison is base (ba03a36) 61.29% compared to head (70e6a18) 61.30%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #332      +/-   ##
==========================================
+ Coverage   61.29%   61.30%   +0.01%     
==========================================
  Files         157      157              
  Lines        9537     9537              
  Branches     1232     1232              
==========================================
+ Hits         5846     5847       +1     
  Misses       3432     3432              
+ Partials      259      258       -1     

Impacted Files	Coverage Δ
balsam/platform/compute_node/nersc_perlmutter.py	33.33% <66.66%> (ø)
balsam/platform/app_run/__init__.py	100.00% <100.00%> (ø)
balsam/platform/app_run/perlmutter.py	29.41% <100.00%> (ø)
balsam/platform/compute_node/__init__.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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