More versatile apps

Slurmer works with apps, that simply consists in an access token an, internally, a directory containing job data and sbatch template.

The parameters used by the sbatch template are fixed, defined in the code, and only cover a very small subset of the sbatch options. We should either include more sbatch options (e.g. GPU numbers, memory, time, partition...), or let the app config defined which options can be processed in the slurmer API requests (most versatile, but maybe less secure?).

Also, I don't see any use case to the chdir option in slurmer, as the template is generated at a time when the job specific subdir is unknown...