fix bug in PoissonSolveFFT that incorrectly inferred the simulation d… #131

Job	Run time
Initial checks	3m 29s
Build documentation	12m 39s
Run unit tests (Julia v1.9)	8m 43s
Run integration tests (Julia v1.9)	9m 49s
Run unit tests (Julia v1.10)	7m 44s
Run integration tests (Julia v1.10)	8m 16s
Run unit tests (Julia v~1.11.0-0)	13m 12s
Run integration tests (Julia v~1.11.0-0)	13m 47s
	1h 17m 39s

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