Geometry File
XFDTD uses two file types for any simulation.  The Geometry file contains information about grid spacing and size, material types, and the location of objects in the grid.  Its
extension is ".id".  The other file type is the fdtd file.  This file contains all simulation
paramenters, including the location of sources, number of timesteps, and data to be saved
during the simulation.
To run a simulation, a Geometry file has to be created.  After the Geometry file has been
saved, an fdtd file needs to be created.  The fdtd file automatically references the geometry
file that was open when it is created.  After the fdtd file is created and saved, calcfdtd
can be run.  The data will be stored in the directory that xfdtd was opened in.  After the
simulation run is complete, the fdtd file needs to be reloaded into xfdtd for data to be
viewed and plotted.