Move PUMAS to externals in atmospheric_physics (ESCOMP#271)

Originator(s):  cacraigucar

Description (include issue title and the keyword ['closes', 'fixes',
'resolves'] and issue number):
 - Introduce PUMAS into atmospheric_physics

List all namelist files that were added or changed: N/A

List all files eliminated and why: N/A

List all files added and what they do: N/A

List all existing files that have been modified, and describe the
changes:
(Helpful git command: `git diff --name-status
development...<your_branch_name>`)
M       .gitmodules
             - Add PUMAS to atmos_phys externals

List all automated tests that failed, as well as an explanation for why
they weren't fixed: none

Is this an answer-changing PR? If so, is it a new physics package,
algorithm change, tuning change, etc? No

If yes to the above question, describe how this code was validated with
the new/modified features:

Author: cacraigucar

.gitmodules

@@ -4,3 +4,9 @@
 	fxtag = 20240626-MPASv8.2
 	fxrequired = AlwaysRequired
 	fxDONOTUSEurl = https://github.com/NCAR/MMM-physics.git
+[submodule "pumas"]
+        path = schemes/pumas/pumas
+        url = https://github.com/ESCOMP/PUMAS
+        fxrequired = AlwaysRequired
+        fxtag = pumas_cam-release_v1.39
+        fxDONOTUSEurl = https://github.com/ESCOMP/PUMAS