Better handling of geometry importing / exporting

[PashavanBijlert]

• [x] Importing of different filetypes (vtp #37 and stl )
• [x] More robust error handling of geometry that is not present
• [ ] Export filetypes? Or only as obj?

[PashavanBijlert]

vtp and stl filetypes and error handling added in v0.7.5. Exporting not yet supported, will likely not support this for vtp.

[PashavanBijlert]

Export filetype flexibillity will be added in a later update, not v0.8.0