Add ChMaterialSurfaceDEM, a class defining material properties for penalty method (DEM) contact.
Add simple demo for DEM contact

Ball bouncing off a box
Cleanup contact force calculation.

Refactor the ChContactDEM class for clarity, improved efficiency, and memory usage.
Irrlicht demos using DEM contact.
Clean-up.

Use the DEM solver (which is just SOR but skips any unilateral constraint work),  but make sure to set non-zero number of maximum iterations (for the bilateral constraint)
Refactor DEM contact to support various force contact models.
Merge branch 'develop' into feature/dem
Merge branch 'develop' into feature/dem
Add ChBodyDEM constructor that specifies the collision model.
Obsolete ChModelBulletDEM.

Instead, use ChModelBulletBody for a ChBodyDEM.
Merge branch 'develop' into feature/dem
cleanup.
Remove obsolete assertion.
Merge branch 'feature/dem' into develop
Fixed names of included header files.
Fix for GCC compilation.
Merge branch 'develop' of https://github.com/projectchrono/chrono into develop