Ensure that VTK is compatibile with Real.

All the VTK recorders as their argument take only double type. But this is only for viewing the results, so I don't think we need to send patches to VTK so that they would accept other precision types. I suppose it is enough if during recording of the results we just convert Real to double.

So in this merge request I introduced macros which help in doing just that. And when compiling with regular double as Real they are completely optimized away, because the static_cast<double> does nothing when given a double argument.

It is ready to be merged.