Add an option, `use_srun: bool`, that can run the Python function without `srun`

[Andrew-S-Rosen]

Closes #62.

### Added

- A new kwarg, `use_srun`, that allows for the user to specify whether to use `srun` when running the pickled Python function.
- Added docstring for `sshproxy`

• [x] I have added the tests to cover my changes.
• [x] I have updated the documentation, VERSION, and CHANGELOG accordingly.
• [x] I have read the CONTRIBUTING document.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.15 :tada:

Comparison is base (8833723) 84.43% compared to head (a60251d) 84.59%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #63 +/- ## =========================================== + Coverage 84.43% 84.59% +0.15% =========================================== Files 2 2 Lines 302 305 +3 =========================================== + Hits 255 258 +3 Misses 47 47 ``` | [Impacted Files](https://app.codecov.io/gh/AgnostiqHQ/covalent-slurm-plugin/pull/63?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AgnostiqHQ) | Coverage Δ | | |---|---|---| | [covalent\_slurm\_plugin/slurm.py](https://app.codecov.io/gh/AgnostiqHQ/covalent-slurm-plugin/pull/63?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AgnostiqHQ#diff-Y292YWxlbnRfc2x1cm1fcGx1Z2luL3NsdXJtLnB5) | `84.53% <100.00%> (+0.15%)` | :arrow_up: |

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[Andrew-S-Rosen]

@wjcunningham7 (and also pinging @rwexler here who brought this up to me originally): I think my work on this one is done, pending review!