Convert integer cast to round() for safety

[heshpdx]

Fix for a verification error on AMD EPYC system using a Cray compiler version 15. Building with -Ofast exposed the problem.
Using these inputs:

axial z-ray separation:            0.25
assemblies per axial sub-domain:   20
reactor height:                    400.00

On the system under test, the I->z_stacked was 79.999992 which was casted down to 79. Using round() fixed this so I->z_stacked computes to 80.