Add 120 task (and maybe 160 task) decomposition for gx3v7

### Description of the issue:

The UCI supercomputer has 40 cores per node, and I set up an 80 task decomposition to spread POP across 2 nodes when running `gx3v7`; they have tried running with 120 and 160 nodes, and run into some issues (see bottom of this section). The auto-decomp tool doesn't provide a very good distribution for those task counts at this resolution, so I'd like to add

```

1
10
10
10
12
cartesian

1
10
8
10
16
cartesian

```

to `bld/generate_pop_decomp.xml` at some point.

The "issues" I've alluded to only seem to be present on their machine, which is using intel 2018.0.3 - there is a crash in `running_means_mod.F90` when `ladjust_bury_coeff = .true.`, but I can't reproduce it on any other machine I have access to. I'm hopeful that it is a compiler bug and updating to a more recent compiler will make it go away, but if it persists I'll open a new issue ticket (I have some ideas on how to investigate if I need to).

### Version:
- CESM: 2.2.0, but working on moving to the latest 2.3 beta tag (waiting on ESMF library on their machine)
- POP2: this just needs to go in the latest, no need to add to 2.2 release tags

### Machine/Environment Description:

GreenPlanet (UCI super computer)

### Any xml/namelist changes or SourceMods:

n/a