file writing for dual_time_stepping_2nd_order

[bigfooted]

Is your feature request related to a problem? Please describe.
2nd order dual time stepping needs restart files for 2 subsequent iterations. If we write files every 100 iterations, we get restart_100.dat and restart_099.dat. However, we also get these additional files for paraview visualization files (i.e. paraview_100.vtu and additional paraview_099.vtu). This is not necessary, leading to a lot of additional files. Furthermore, we usually want more visualization files then we want restart files.

Describe the solution you'd like
Less files for unsteady simulations by:

1. introducing different writing frequencies for different files
2. not writing the visualization files for N and N-1 (do we ever need this?)

How could we do this? One idea: for every file type in OUTPUT_FILES we have to additionally provide the writing frequency. e.g.
OUTPUT_FILES= (RESTART, PARAVIEW)
OUTPUT_WRT_FREQ= (5000, 10)

Or even: OUTPUT_FILES= (RESTART, 5000, PARAVIEW, 10)