Miscellaneous warning fixes (mostly related to type conversions)
On windows, disable warning 4275 when building the Irrlicht demos.
Fix implementation file for ChLcpMatlabSolver

For some reason, this was a copy of the header file ChMatlabEngine.h!!
Fix compilation of FEM demos which use Irrlicht and Matlab.
Downcasting of shared pointer in demo_FEMvisualize.cpp.
(This is needed since the new implementation of shared pointers do not provide automatic downcasting of pointers, in sake of a more strict enforcing of type checking from the user side.)