Average low, mids and high bed occupancy predictions

[thibautjombart]

Currently we have n_sim or each type of predictions, which creates problems to then plot things and show results.

We need to

• [x] average predictions by type and only output these
• [x] change the plotting function to accommodate this change

[thibautjombart]

This was done at 7089ef130c36c9a219e4b50fa7c29a99724e0e21 but solution is abandonned.