Minor CMakeLists cleanup for vehcle checkpoint demos

Some fixes for ChSystem copy constructors

Patch assimp before building the VSG dependencies (fix for newer versions of clang)

Install the `yaml-cpp` target from top-level CMakeLists.txt.

Use an internal chached CMake variable to keep track of YAML support

Change `enable_YAML` from CMake macro to function

Change configuration of YAML parser support (via yaml-cpp).

fix: use ImportCheckpoint for tires in demo_VEH_Checkpoint SimulateBoth (#723)

Fix typos

Fix use of std::abs in Mat33 constructor (for the no AVX and no SSE option)

Clean up and streamline functions for multicore math.

Fix initialization of variables in ChSolver constructor

Add DGX Spark Runners to CI/CD pipeline (#722)

Refined utest_MOD_curved_beam

Added demos and utest for saving and loading reduced model