WIP: Feature/example rot sym

Fixes #787 (closed)

ToDos:

• Update documentation, especially in spatialparams
• explain rotationsymmetricgridgeometrytraits
• write 1p equation in radial coordinates and specify analytical solution
• generate and include result pictures
• maybe also explain how the paraview filter can be used for visualization
• Move calculate L2 error to main.cc
• Move add vtk output to main.cc