Feature Request: `update` command

[n-gao]

Follow-up from #81.
Add a new command that allows one to update pending jobs by overriding their sbatch and config parameters.

Proposed syntax:

seml <collection> update -b 7 -sb mem=25GB -o dataset=imagenet

Considerations:

• What happens if the update results in identical configurations?
• Should one only be able to update sbatch params on a batch basis? - Needed for multi-gpu support and slurm batching.