Add DEM prototype

[nathanfranklin]

This adds a DEM prototype which uses FILE-IO wrapper that in turn uses LIGGHTS

[nathanfranklin]

@ahashibon , here is where I am working on a DEM prototype that extends our current MD wrapper to support DEM (using LIGGHTS)

[nathanfranklin]

@ahashibon , @TobiasRasp , here is an update on extending this wrapper to support DEM through LIGGGHTS

Currently, the file utilities (read_data_file and write_data_file..) have been extended to support different atom types (currently "atomic" and "granular"/"spherical"). In addition, the wrapper has been extended so that it can be either configured for MD or DEM. The DEM wrapper assumes values for some things so that we can run a single example (dem_billiards.py).

what still is missing is:

• testing of DEM wrapper
• documentation
• complete configuration (YOUNGS_MODULUS, POISSONS_RATIO, and material relations for coefficientrestitute and coefficientFriction)
• support walls
• DEM wrapper using INTERNAL interface

I put these in order of when I will do them. I suspect the last one has a low priority

EDIT: created an issue to track these todos (#104)