Prevent very intensive computations (or allow to stop them)

[daemontus]

Currently, rendering of big graphs or computing non trivial data (like transitions) cannot be interrupted by the user. So by uploading a very large model, you can overload the server without the ability to recover.

Possible solutions:

• Have an upper bound on model complexity and refuse models which are too big.
• Make such operations interruptible and terminate them when the session ends