CGT: weighting matrices names changed:

- S -> W, generated in acadoVariables
- SN -> WN, generated in acadoVariables

This is deliberately done because Simulink coder
is not happy with structure fields with name "S".

This is not backwards compatible and it is going
to break old codes.

Generator changes are only in ExportNLPSolver
class. Examples and Matlab+Simulink interfaces
are updated accordingly.