Bugfix in 1d model output

 - Value UNITS was not written out in model_discontinuities.f90
 - number of columns was wrong in background_models.F90
Merge branch 'master' of https://github.com/geodynamics/axisem
adding a comment on vtk/xdmf
Merge branch 'master' of https://github.com/geodynamics/axisem
Update .gitignore
Manual: detailed description of external models
formatting of error msg
adding a vtk file with element type
changing format for Q to be able to write values of 1e6 properly
beautification
removing unused function + beauty
removing commented function, special version for npol=4 seems not to be necessary for

performance
removing useless consistency check
starting to implement displacement only option for kernel wavefield output
produce backtrace in check()
beauty
beauty
removing unused function
preparations for dump_type displ_only
working on output for displ_only mode, not running yet