Will not open 3MF files

[gzraick]

#Please read carefully before submitting an issue

• be as detailed as possible while describing an issue:
  • which platform are you using (say it also on the subject of the issue)
  • which version of meshlab are you using (e.g. 2020.09 portable. 2020.06 snap, 2020.12 installer)
  • be sure that the issue is reproducible: list step by step all the operations performed to reproduce the issue
  • if it applies, share all the involved files
• if it is possible, verify if the problem persists in other platforms;
• be sure that your issue also happens on the most recent MeshLab version;
• check if your issue has been already reported by someone else. In this case, do not open another issue: just add your experience in the existing issue;
• delete these guidelines before submitting :)

If it is not an issue but something else (need help, feature request, ...) consider to use github discussion: https://github.com/cnr-isti-vclab/meshlab/discussions

[alemuntoni]

See #1032.