Fix bug in perturbation.py (#525)

### Description
Fix the bug in perturbation.py to compare `f.solenoidal` to both `None`
and `"None"`

### Related issues
Pointed out by sonarcloud

### Checklist
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- [x] I have added a description (see above).
- [ ] I have added a link to any related issues see (see above).
- [x] I have read the [Contributing
Guide](https://github.com/quokka-astro/quokka/blob/development/CONTRIBUTING.md).
- [ ] I have added tests for any new physics that this PR adds to the
code.
- [ ] I have tested this PR on my local computer and all tests pass.
- [ ] I have manually triggered the GPU tests with the magic comment
`/azp run`.
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---------

Co-authored-by: Piyush Sharda <[email protected]>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Piyush Sharda <[email protected]>
Co-authored-by: Piyush Sharda <[email protected]>
Co-authored-by: Ben Wibking <[email protected]>
Co-authored-by: Piyush Sharda <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/perturbation.py

@@ -273,7 +273,7 @@ def plot_spectrum1D(pertx, perty, pertz):
 # data precision
 dtype = np.float64
 # ratio of solenoidal to compressive components
-if options.f_solenoidal is None or options.f_solenoidal is None:
+if (options.f_solenoidal is None) or (options.f_solenoidal=="None"):
  f_solenoidal = None
 else:
  f_solenoidal = min(max(float(options.f_solenoidal), 0.), 1.)