StsSetup: replace BBox recompute fct w/ one proposed by ROOT expert, #2620

• cf https://github.com/root-project/root/issues/12242
• Fixes edge case of #2620 which was missed by previous patch

Problem description (also valid for similar patch in Fairroot v18.8.0 and v18.8.1):

1. previous re-computation patch is only applied to Assemblies which are Physical Nodes (first check if PhysicalNode, then if Assembly)
2. by default, Assemblies do not have/are not PhysicalNodes
3. the alignment procedure forces a PhysicalNode creation/conversion only on the nodes being aligned

=> If only lower levels inside the assemblies (e.g. Sensors but no Station) were aligned, the Assemblies were not recomputed

Handles problem reported by @a.bercuci without needing fix/trick on the user side
Needed until we deprecate support for FairRoot v18.6.x (similar fix done in FairRoot from v18.8.2 on)
Could also be deprecated if the handling is improved on the ROOT side, but timescale of our Fairsoft/Root transitions typicaly even longer than our FairRoot transitions