#98 Add support for Nexus files

[Jesse Hopkins]

Javier sent me some SWING Nexus files, which I will attempt to attach (may be too large). I would like to add support for reading in this format. He recommended building a definitions file that lives separate from the program code. Then a user can create new definitions files as desired, and the program doesn't have to be recompiled (or have new code added) in order to use this definitions. Makes it much easier to extend it for users.