Clean up default setting for OCN_CHL_TYPE

### Description of the issue:

For production runs, we want `OCN_CHL_TYPE=prognostic` if POP is running with the ecosystem tracer. For testing purposes, it would be useful to have a compset modifier to switch back to `diagnostic` in https://github.com/ESCOMP/POP2-CESM/blob/master/cime_config/config_component.xml#L37..L54

So something like

```

char
diagnostic,prognostic
diagnostic

prognostic
diagnostic

run_component_pop
env_run.xml

Determines provenance of surface Chl for radiative penetration,
computations. This option is used in the POP ecosystem model.
The default is diagnostic.


```

and then update `G` compsets we use in the test suite to be `POP2%ECO%DIAG_CHL` if we want to keep the tests bit-for-bit.

### Version:
- CESM: latest
- POP2: latest (`master` as of `cesm_pop_2_1_20200709`)