NetCDF global attributes incomplete

I was looking at an EAMxx history file and noticed a number of the global attributes are potentially misleading. For example, IC and topo files were used in the simulation, but here they show as "NONE FOR NOW". The case and git_hash attributes would also be useful.
```
// global attributes:
:source = "E3SM Atmosphere Model Version 4" ;
:case = "TEST 1" ;
:title = "EAMv4 History File" ;
:git_hash = "THE GIT LOG HASH" ;
:host = "THE HOST" ;
:Version = "1.0" ;
:revision_Id = "None" ;
:initial_file = "NONE FOR NOW" ;
:topography_file = "NONE FOR NOW" ;
:contact = "e3sm-data-support@llnl.gov" ;
:institution_id = "E3SM-Project" ;
:product = "model-output" ;
:realm = "atmos" ;
:Conventions = "None yet" ;
```

Aaron Edit: Adding a checklist for all of the attributes we want to add:

- [x] source
- [x] case
- [ ] title
- [ ] git_hash
- [x] host
- [ ] version
- [ ] revision_id
- [ ] atm initial_file
- [ ] lnd initial file
- [x] topography_file
- [x] contact
- [x] institution_id
- [ ] product
- [x] realm
- [x] Conventions
- [x] averaging type
- [x] output frequency
- [x] username who submitted run
- [ ] time written
- [x] time_period_frequency (see Rob's comment below, note we also want to add `cell_methods` to each variable)