Improving the documentation/interface of the t-out-of-N-threshold scheme

[ChristianMct]

As discussed in the committee meetings, the current t-out-of-N-threshold scheme is insufficiently documented and lacks an end-to-end example. This PR will fix this by:

1. improving the README.md in the multiparty package:
   • Add a high-level description of the $N$ -out-of- $N$ -threshold scheme
   • Add a high-level description of the $t$ -out-of- $N$ -threshold scheme
   • Adapt the overview of the MHE-MPC protocol
2. implementing a new example or, potentially, converting the int_pir example to a $t<N$ setting.
3. if needed, consider API changes to ease the use of the $t$ -out-of- $N$-threshold scheme.

Current Status:
This PR is a working draft, but please feel free to start the discussion on any of the point above.
For now, I have pushed drafts commits that cover points 1 and 2.