listmode Hessian test fails on MacOS CLang 18

[KrisThielemans]

https://github.com/UCL/STIR/actions/runs/10450627407/job/28935385772?pr=1489#step:12:207

----- testing concavity via Hessian-vector product (accumulate_Hessian_times_input)

INFO: Loaded 8019 prompts from list-mode file

INFO: Caching Additive corrections for : 8019 events.

INFO: Listmode gradient calculation: starting loop with 1 thread

INFO: Computation times for distributable_computation, CPU 0.01s, wall-clock 0.001722s

INFO: Loaded 8019 prompts from list-mode file

INFO: Caching Additive corrections for : 8019 events.

INFO: Listmode gradient calculation: starting loop with 1 thread

INFO: Computation times for distributable_computation, CPU 0s, wall-clock 0.001746s
Error : 0 is larger than 0, 
FAIL: PoissonLLListModeData: Computation of x^T H x = 0.000000 > 0 (Hessian) and is therefore NOT concave
 >target image max=0.999849
 >target image min=1.16359e-05
 >output image max=0
 >output image min=0
Writing diagnostic files to PoissonLLListModeData_concavity_out.hv, *target.hv

Other tests seem to be fine

[KrisThielemans]

Possibly related to #1200

[KrisThielemans]

Identical error seen on AppVeyor build for a private version of STIR, for both Visual Studio 17 2022 and Visual Studio 16 2019. We don't have this on the public version of STIR appveyor builds. As far as I can see, the difference between those is that the public version uses Appveyor Build worker image Visual Studio 2015, while the private one uses Build worker image Visual Studio 2019. (I'm not 100% sure what the consequences of this are, but presumably it means that underlying software is more recent on the 2019 version.)