Regroup demos and fix numpy compatibility issue

[chaozg]

Hi @jakobsj, I'm resolving three issues in this one PR, because:

• we need Compiling with the latest version of NumPy fails #729 (to fix numpy compatibility issue) to run CUQIpy tests, otherwise many tests (irrelevant to this PR) will fail;
• Group some demos to unmaintained #696 involves only file relocation and Gibbs tutorial needs update #730 needs some tiny code changes and relocation: these two very closely related, so I fixed them two also together.

fixed #696
fixed #729
fixed #730

[jakobsj]

Hi @chaozg thanks for this nice PR cleaning up and organizing the old demos. All looks good, just one small question about the change in numpy version requirement, please see the inline comment.