Improve the `ProcessMonitor`

[superstar54]

The ProcessMonitor class monitors a process by executing callback functions at specified intervals as long as the process is alive.

We may extend it to cover other use cases, for example:

1. Only when a process is finished, run the callback functions
2. Only when a process changes its state, run the callback functions
3. Only when a process changes its outputs, run the callback functions

[danielhollas]

I believe point 1. is already implemented via the on_sealed argument.

[superstar54]

Hi @danielhollas , Thanks very much for pointing out this. I didn't notice this because of the name of the variable callbacks, we may need to change it to on_running.

[danielhollas]

To clarify, on_sealed callbacks are run exactly once when the process finishes (i.e. when the corresponding node in aiida DB is sealed, made readonly). Arguably the name exposes an implementation detail of AiiDA and could be named on_finished or something, but I think having a good documentation for this should suffice instead of making breaking change.

[danielhollas]

Ah, I may have misunderstood your comment, I see what you mean now.

[superstar54]

We may add more categories:

• on_running, the callbacks used by now.
• on_sealed
• on_finished_ok
• on_entered (for a new state)
• on_modified