Adding material.get_element_atom_densities

[shimwell]

Description

We have a openmc.Material.get_nuclide_atom_densities and this PR aims to add openmc.Material.get_element_atom_densities

This would be useful for @rherrero-pf and myself when we perform DPA simulations we need to get the atom density for elements.

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 @shimwell! I did a little clean up as usual 😄

[shimwell]

Thanks Paul.