Fox submodule

FoX XML is now a git submodule in OpenMC. The submodule can be fetched manually (from root repo dir) with
```git submodule init```
```git submodule update```

These commands are also executed from CMakeLists.txt if it can't find the src code for FoX. Developers will have to read up on how to work with submodules if they want to edit FoX's source code.

The following changes were made to FoX and are on the openmc branch of mit-crpg/FoX:
- made Node, NodeList, NodeListPtr public
- edited main CMakeLists.txt in FoX root directory to be compatible with building OpenMC
- long integer scalar support

The following changes were made to OpenMC:
- custom FoX add-ons are now in xml/openmc_fox.F90 which now acts as a buffer between xml_interface.F90 and FoX.