Unnecessary accesses to the APIs' files

[MatthieuNICOLAS]

Currently, we're retrieving the worlds' APIs from files for each instance.
It would be cleaner to retrieve them once and for all during initialization and then to share them among all instances of worlds.

[mquinson]

In the same area, we need a mechanism to ask the PLM to refresh its cache so that modified exercises get reloaded.

Thanks for your work,

[mquinson]

The cache refresh seems fixed to me, but @MatthieuNICOLAS could you please check whether you fixed the first issue you mentioned? If not, you'll need to be more explicit for me to understand what's still to do.
Thanks,