Add nuclear data path to every example

[shimwell]

adding openmc.config['cross_sections'] = 'username/nuclear_data/cross_sections.xml' to every example would help people running it from outside of the docker container

users would also have to download the nuclear data with a command like this

mkdir /home/username/nuclear_data && wget https://anl.box.com/shared/static/uhbxlrx7hvxqw27psymfbhi7bx7s6u6a.xz -O /home/username/nuclear_data/endf.tar.xz && tar -xvJf /home/username/nuclear_data/endf.tar.xz -C /home/$HOME/nuclear_data && cd -

the username would need updating to your own in each command