Add a max_concurrency setting

[rikhuijzer]

This could be nice in cases where the parallel build is causing problems (setting max_concurrency=1) or to reduce the number of concurrent runs when each tutorial spawns many threads.

[rikhuijzer]

This is sort of solved in #37 by setting use_distributed=false.

[rikhuijzer]

Jup this is really important to fix. My pc grinded to a halt on 11 notebooks because it’s using all available memory and swap memory. Need to keep the async task spawning in check