add support for specifying slurm output file

[cnrrobertson]

As seen in #185, if an output argument (-o or --output) is added to the srun arguments, the process fails. This checks to see if an output name has been specified and if not, uses the previous default name format.

[kescobo]

This seems good to me! Tests are failing, but what else is new? Thanks for adding some for this change!

[cnrrobertson]

Yeah, I wasn't sure about the error message on the CI. Seemed like it is related to allocating slurm workers?

 connecting to worker 1 out of 1
srun: error: get_addr_info: getaddrinfo() failed: Name or service not known
srun: error: slurm_set_addr: Unable to resolve "slurmctl"
srun: error: Unable to establish control machine address
srun: error: Unable to allocate resources: No such file or directory

I can tell you it ran on the cluster I was using fwiw.

[kescobo]

Yeah, tests are failing on master too - no worries.

I'm happy to merge in ~a week if no one objects.