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.
Merge branch 'develop' of https://github.com/projectchrono/chrono into develop

Conflicts:
	src/demos/dem/demo_ball.cpp
	src/physics/ChMaterialSurfaceDEM.h