Replaced notifications of the exported integrator to the new style.

The debug info can be enable in the user app with:

Logger::instance().setLogLevel( LVL_DEBUG );
Updated timestamp of the acado_utils file
Working on performance issue while exporting "big" expressions.

There is a performance issue while exporting big functions using
ExportFunction. The issue is related to setVariableExportName function
which is called recursively.

Possible solution is just to call this function in TreeProjection
class only for an argument of type Power_Int.
Cleaning the ExportStatement infrastructure.

Refactored derived classes in order to simplify coding of user classes.

Main improvement is that certain (helper) functions return a reference
to a given class.
Working on decoupling standard ACADO and Code Generation Tool (CGT).

Removed dependency of the standard ACADO on CGT while exporting ACADO
symbolic functions, i.e. removed dependency on ExportVariable.

Specific getter/setter functions are moved to the ExportODEFunction now.

BTW, we should rename ExportODEFunction to ExportACADOFunction.