Make appending a timestamp to rpointer files optional

[dougiesquire]

This PR allows writing restart pointer files without a timestamp in the filename by setting a flag in the NUOPC runconfig file (restart_pointer_append_date = .false.). The default behaviour if restart_pointer_append_date is not set is to include a timestamp in the filename.

See mom-ocean#1653 (review) for motivation.

[jedwards4b]

@alperaltuntas we will need a change in cmeps to create the restart_pointer_append_date in order for this to work and it's not clear to me why
the cesm_coupled variable doesn't work.

[jedwards4b]

It occurs to me that we already have a variable in the nuopc.config which will provide the information about whether rpointers have timestamps - that is the variable drv_restart_pointer - if this variable is present then MOM should use timestamp on the rpointer file. Can this PR just use the existence of that variable instead of introducing a new variable?

[dougiesquire]

Thanks @jedwards4b. Unfortunately I'm not sure drv_restart_pointer will work for us. We use CMEPS with the CESM driver but want to be able to retain the old behaviour of having pointers without timestamps. In our nuopc.runconfig we have:

DRIVER_attributes::
     ...
     drv_restart_pointer = rpointer.cpl
     ...

[alperaltuntas]

@jedwards4b, Since @dougiesquire set pointer_date to .true. by default, no changes are needed in CMEPS. I ran an ERS test case with this PR, without modifying CMEPS, and confirmed that the test passed, with the date suffix correctly added to the rpointers. However, if you still recommend adding restart_pointer_append_date to CMEPS, let me know and I can submit a PR to CMEPS.

[jedwards4b]

@alperaltuntas If you can solve the issue without changes to cmeps, please do.