Adding bounding box to spherical mesh

[caderache2014]

Description

Adapted computation of lower_left and upper_right coordinates for spherical mesh objects. Tested against BoundingBox class

Fixes # 2550

• I have performed a self-review of my own code
• I have run clang-format on any C++ source files (if applicable)
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• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[caderache2014]

@shimwell I think something went awry with this pull request. I don't see any of the changes we made to the SphericalMesh class. No wonder the CI test is failing. There is no bounding_box property in this version of SphericalMesh. I'm trying to reconstruct the changes. I'll submit a new pull request

@Property
def bounding_box(self):
return openmc.BoundingBox(
np.array(self.origin[0] - self.r_grid, self.origin[1] - self.r_grid, self.origin[2] - self.r_grid),
np.array(self.origin[0] + self.r_grid, self.origin[1] + self.r_grid, self.origin[2] + self.r_grid)
)

[shimwell]

thanks for following this up @caderache2014

closing this PR in favor of #2620