Adds slurm support

[o-smirnov]

@SpheMakh, in the interest of smaller PRs, review early, review often please!

@Athanaseus this ought to get you going. The slurm backend "wrapper" is enabled like so:

stimela/tests/stimela_tests/test_slurm.yml

Line 11 in e61296d

slurm:

This causes all commands to be wrapped in an srun call. Just add scatter: -1 to your for-loops, and you're all parallel.

(I call it a "wrapper" because it wraps an actual backend like "native" or "singularity" -- it is not a backend on its own per se.)

The recipe itself is meant to be run on the head node (inside a tmux or screen session, of course).

Under slurm, you don't want to be building Singularity images on-demand on the compute nodes. Rather, pre-build all necessary images on the head node before you run a recipe. There is a new stimela build recipe.yml command that will do this (options very similar to the run command). You should also disable on-demand building of images during the run like so:

stimela/tests/stimela_tests/test_slurm.yml

Line 10 in e61296d

auto_build: false # on slurm, we pre-build on the head node explitictly

You'll probably want to tweak srun options on a per-cab (or per-step) basis. This is done similarly to how we tweak k8s settings per cab here:

stimela/tests/stimela_tests/test_kube_config.yml

Line 42 in e61296d

## some cab-specific backend tweaks

So, something like:

cabs:
  breizorro:
    backend:
      slurm:
        srun_opts:
          mem: 16GB
          cpus_per_task: 4 

..will run breizorro with srun --mem 16GB --cpus-per-task 4. The srun_opts section is free-form, so
all srun options are supported.