replace loader assertion by error messages
move Loader to core/io/, move data transfer from vectors to VTK format to MatricesToVtk
support loading of images/3D grids from VTK XML image format
Merge remote-tracking branch 'origin/master' into backend_integration
remove VTK dependency from SimulationWidget

* read input files with file_parser
* require simple text files as input, instead of files with header/meta data