Fourier sublattice bug

[mducle]

Fixes a parsing error in the spinw.fourier method which occurs if the sublat option is not given.

As the function is rarely used and so is not tested this was not caught before.

It seems that originally there was no sublattice option, it was introduced in order to work with a new option in the developmental scga method but the original use (e.g. in scga3) was not tested at the time (Aug 2017).

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (ea5ea33) 40.48% compared to head (6835ab2) 40.51%.

Files	Patch %	Lines
swfiles/@spinw/fourier.m	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #153 +/- ## ========================================== + Coverage 40.48% 40.51% +0.02% ========================================== Files 240 240 Lines 15979 15981 +2 ========================================== + Hits 6469 6474 +5 + Misses 9510 9507 -3 ```

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[github-actions[bot]]

Test Results

       4 files  ±0     106 suites  ±0   14m 26s :stopwatch: - 3m 36s    619 tests ±0     618 :heavy_check_mark:  - 1  0 :zzz: ±0  1 :x: +1  1 766 runs  ±0  1 764 :heavy_check_mark:  - 2  0 :zzz: ±0  2 :x: +2 

For more details on these failures, see this check.

Results for commit 6835ab26. ± Comparison against base commit ea5ea334.

:recycle: This comment has been updated with latest results.

[RichardWaiteSTFC]

Merging as I think the failure is a flaky test, perhaps more flaky on R2023b?