Dynamic generation of met_data_file names prevents check_input_data from downloading needed data

This issue started in the forums:

https://xenforo.cgd.ucar.edu/cesm/threads/issue-downloading-merra-files-during-testing.4952/

The problem seems to be that CAM's namelist lists a single `met_data_file`, e.g.

```
met_data_file = '2000/MERRA2_1.9x2.5_20000101.nc'
```

and then other file names (with other dates) are generated dynamically at run time.

However, this means that `check_input_data` does not know about these other files, so they are not automatically downloaded (even though they do exist in the input data repository).

This should be changed in some way to allow check_input_data to work properly, downloading all needed files for a case. Two options that come to mind are:

(1) All file names can be generated and listed at build-namelist time

(2) check_input_data can be enhanced to understand that, in some cases (like this one) it needs to get all files from a given directory, rather than just the specific listed file.

I have reproduced this error both in CESM2.0.1 (where the user reported the issue) and in a recent version of the cesm2.1 alphabranch. I reproduced it by running

`./create_test --no-build SMS_Ld1.f19_f19_mg16.FXSD.bishorn_gnu.cam-outfrq1d` (where bishorn is my local machine), then running `./preview_namelists` followed by `./check_input_data`, and noting that only one of the MERRA files is considered by check_input_data.