Fix for depletion with photon transport turned on

[paulromano]

This PR fixes #1585. Depletion relies on the openmc_load_nuclide C API call, and currently this function only loads incident neutron data. I've changed it so that it will now load photoatomic data as well. I've also refactored things a bit to improve the structure:

• cross_sections.cpp now uses openmc_load_nuclide as well to reduce redundant code
• Adding entries in nuclide_map and element_map is handled inside of the Nuclide and PhotonInteraction constructors
• Determining the min/max energies for neutron/photon transport and setting up the log grid for energy searches is determined during simulation_init rather than when cross sections are loaded
• data::element was set up as a vector of PhotonInteraction objects rather than pointers. This means that a push_back could result in all the photon data being copied. I've changed it to a vector of unique_ptr so that only pointers need to be copy upon a vector resize.

[amandalund]

Just a small documentation fix, otherwise everything works as expected! Thanks @paulromano.

[amandalund]

This link won't render correctly because of the nested backquotes.

[paulromano]

Thanks for pointing that out! Just fixed it