cylindrical mesh lower left Z value corrected

[shimwell]

Description

I think the CylindricalMesh.bounding_box.lower_left Z value might not be correct.

Steps to reproduce

import oenmc
mesh = openmc.CylindricalMesh(
    r_grid=[1,2,3],
    z_grid=[0,10,20,30]
)
print(mesh.bounding_box)
BoundingBox(lower_left=(-3.0, -3.0, -30.0), upper_right=(3.0, 3.0, 30.0))

I was expecting the result to be
BoundingBox(lower_left=(-3.0, -3.0, 0.0), upper_right=(3.0, 3.0, 30.0)) but it currently returns -30 instead of 0 for the lower left z value. As the mesh.origin is 0,0,0 and the z_grid goes from the origin upwards in steps of 0,10,20,30. There is no negative value in the z grid so I think the lower left value is not calculated correctly at the moment

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Thanks for the fix!