Avoid duplicate materials written to XML

[RemDelaporteMathurin]

Description

Recently, we accidentally added the same Material object to a Model twice which made the simulation crash because there were duplicate IDs in the XML. Consider the following MWE:

import openmc

my_mat = openmc.Material(name="my_mat")
my_mat2 = openmc.Material(name="my_mat2")

materials = openmc.Materials([my_mat, my_mat2, my_mat])

materials.export_to_xml("materials.xml")

materials_in = openmc.Materials.from_xml("materials.xml")
assert len(materials_in) == 2

A simple way to get rid of this is to only export unique entities to XML.

@pshriwise

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• 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)

[paulromano]

Thanks @RemDelaporteMathurin!