Add the option to increase output resolution.

deal.II offers to interpolate higher-order cell output to a finer output mesh. This actually represents the internal accuracy much better than the usual output only at the cell corners (see attached figure, same model, different output resolution). Should we offer the user an option for this in the visualization postprocessor? At least I am pretty sure I would always prefer the higher resolution output, even when I know it is only a bilinear interpolation of the quadratic element functions. I was always annoyed that aspect output with the same number of degrees of freedom looks much coarser than Citcom output in Paraview, although the internal resolution is similar ;-) 

![image](https://cloud.githubusercontent.com/assets/7582930/4263642/bb0b8254-3bf5-11e4-9d2c-f8950f85346c.png)