Add JEFF 3.2 cross sections

[shimwell]

Neutrons for the JEFF 3.2 distribution have been processed in this repo https://github.com/openmc-data-storage/JEFF-3.2

However they need adding to the cross_sections_directory.py file like the other h5 file available.

This can be done with a large pull request or several smaller ones to the develop branch:

• the introduction of a new variable called jeff_32_neutron_isotopes in the cross_sections_directory.py file . The varible should contains all the isotope names in this folder

• then a small section that builds up the dictionary of information on the cross section like I've done for other neutron libraries for example

• then the add the new collection to the existing collections on this line

• then change the README.md file so that it has a few JEFF-3.2 examples

• Add some tests to here and here