New classes for Gauss-Legendre quadrature of functions in 1D, 2D, 3D intevals.
Will be used for advanced FEM.
Note that the ChQuadrature class, thanks to templating, can also integrate tensorial/vectorial functions.
The quadrature algorithm is optimized so that, in case of low (<10) order, precomputed polynomial coefficients are used, otherwise, in sake of generality, for larger orders it automatically falls back to just-in-time polynomial table computation.
Add tangential damping coefficient.
Obsolete ChSystemOpenMP.
Remove use_GPU flag.
Make ChSystem::Integrate_Y() virtual.
Clean up the falling ball DEM demo.
Add compiler specific flags for darwin
Not used right now but clang specific stuff can be added in the future
Clang does not like ComputeEigenValues, need to provide base class
Find Irrlicht on darwin like we do for linux
Merge pull request #15 from jcmadsen/develop

ChQuaternion<Real> GetInverse() const       throws an error when used outside of the ChronoEngine build
Add ChSystemDEM, a thin class derived from ChSystem

This allow controlling the solvers and contact system that are meaningful for DEM simulations.
Cleanup.
Updated to use ChSystemDEM.

Also, include the (DEM) bodies in the system's list of bodies (as opposed to the list of 'other physics').
Merge branch 'feature/dem' into develop
Refactor DEM contact force calculation.
Cleanup DEM demos.
Use texture asset.
Cleanup of DEM classes.

- obsolete structure ChContactKinematicsDEM
- renamed member variables in ChCompositeMaterialDEM
- use the minimum value of mu as the coefficient of friction for a pair
Removed 'dem' demo directory
Include <cmath> for definition of std::sqrt and std::abs.