Change of structure for v1

In order to hide viz.itk_vtk_viewer.NotInstalledError and similar classes, I moved the code from cli into a function in viz module and only this function is imported for cli. https://www.notion.so/qim-dtu/v1-0-prep-itk-vtk-viewer-15abab83b2f980c485caf84a938faaa1

requested review from @fima

assigned to @s233039

approved this merge request

merged

mentioned in commit 31fba17b